diff --git a/v2/Cargo.toml b/v2/Cargo.toml
index c285f9f6..b6a11087 100644
--- a/v2/Cargo.toml
+++ b/v2/Cargo.toml
@@ -28,6 +28,7 @@ members = [
     "crates/wifi-densepose-geo",
     "crates/nvsim",
     "crates/nvsim-server",
+    "crates/homecore",                 # ADR-127 — HOMECORE state machine
     # ADR-100/ADR-101: Cognitum Cog packaging — first Cog from this repo.
     # Ships the wifi-densepose pose-estimation model as a signed binary +
     # JSONL manifest installable by the Cognitum V0 appliance (cognitum-v0,
diff --git a/v2/crates/homecore/Cargo.toml b/v2/crates/homecore/Cargo.toml
new file mode 100644
index 00000000..2242e6f4
--- /dev/null
+++ b/v2/crates/homecore/Cargo.toml
@@ -0,0 +1,43 @@
+# HOMECORE — Rust state machine, event bus, service registry, entity registry.
+# Implements ADR-127 (HOMECORE-CORE), the foundation of the HOMECORE Home
+# Assistant port (ADR-126 master + ADR-128/129/130/131/132/133/134 sub-ADRs).
+#
+# P1 scaffold (this commit): public types + DashMap-backed state machine +
+# Tokio broadcast event bus + minimal entity registry. Persistence and the
+# full HA-compat serde schema land in P2.
+
+[package]
+name = "homecore"
+version = "0.1.0-alpha.0"
+edition = "2021"
+license = "MIT"
+authors = ["rUv <ruv@ruv.net>", "HOMECORE Contributors"]
+description = "Rust state machine + event bus + service registry — the foundation of the HOMECORE Home Assistant port (ADR-127)"
+repository = "https://github.com/ruvnet/RuView"
+
+[lib]
+name = "homecore"
+path = "src/lib.rs"
+
+[dependencies]
+# Core async runtime — matches the rest of the v2/ workspace (sensing-server etc.)
+tokio = { version = "1", features = ["sync", "rt", "rt-multi-thread", "time", "macros"] }
+# DashMap for the concurrent state store — ADR-127 §2.1.
+dashmap = "6"
+# Typed event channels + service handler boxing
+futures = "0.3"
+async-trait = "0.1"
+# Time types matched to HA's UTC datetime usage
+chrono = { version = "0.4", features = ["serde"] }
+# Schema validation replacement for voluptuous (ADR-127 §3)
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+# Unique IDs (Context, ConfigEntryId, DeviceId)
+uuid = { version = "1", features = ["v4", "serde"] }
+# Error handling
+thiserror = "1"
+# Read-only static catalogs (event type names etc.)
+once_cell = "1"
+
+[dev-dependencies]
+tokio = { version = "1", features = ["sync", "rt", "rt-multi-thread", "time", "macros", "test-util"] }
diff --git a/v2/crates/homecore/src/bus.rs b/v2/crates/homecore/src/bus.rs
new file mode 100644
index 00000000..661c3606
--- /dev/null
+++ b/v2/crates/homecore/src/bus.rs
@@ -0,0 +1,90 @@
+//! Event bus — typed system events + untyped domain events.
+//!
+//! ADR-127 §2.2: HA's single dict-typed event channel becomes two:
+//! - typed `SystemEvent` channel for known shapes (recorder, automation)
+//! - untyped `DomainEvent` channel for arbitrary integration events
+//!
+//! Capacity 4,096 on both. Lagged receivers must re-sync (recorder
+//! re-reads current state; automation re-evaluates triggers).
+
+use std::sync::Arc;
+
+use tokio::sync::broadcast;
+
+use crate::event::{DomainEvent, SystemEvent};
+
+pub const EVENT_CHANNEL_CAPACITY: usize = 4096;
+
+#[derive(Clone)]
+pub struct EventBus {
+    inner: Arc<EventBusInner>,
+}
+
+struct EventBusInner {
+    system_tx: broadcast::Sender<SystemEvent>,
+    domain_tx: broadcast::Sender<DomainEvent>,
+}
+
+impl EventBus {
+    pub fn new() -> Self {
+        let (system_tx, _) = broadcast::channel(EVENT_CHANNEL_CAPACITY);
+        let (domain_tx, _) = broadcast::channel(EVENT_CHANNEL_CAPACITY);
+        Self {
+            inner: Arc::new(EventBusInner { system_tx, domain_tx }),
+        }
+    }
+
+    pub fn subscribe_system(&self) -> broadcast::Receiver<SystemEvent> {
+        self.inner.system_tx.subscribe()
+    }
+
+    pub fn subscribe_domain(&self) -> broadcast::Receiver<DomainEvent> {
+        self.inner.domain_tx.subscribe()
+    }
+
+    /// Fire a typed system event. Returns the number of active
+    /// receivers (zero is fine).
+    pub fn fire_system(&self, event: SystemEvent) -> usize {
+        self.inner.system_tx.send(event).unwrap_or(0)
+    }
+
+    /// Fire an untyped domain event. Mirrors `hass.bus.async_fire`.
+    pub fn fire_domain(&self, event: DomainEvent) -> usize {
+        self.inner.domain_tx.send(event).unwrap_or(0)
+    }
+}
+
+impl Default for EventBus {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::event::Context;
+
+    #[tokio::test]
+    async fn fire_system_reaches_subscriber() {
+        let bus = EventBus::new();
+        let mut rx = bus.subscribe_system();
+        bus.fire_system(SystemEvent::HomeCoreStarted);
+        let event = rx.recv().await.unwrap();
+        assert!(matches!(event, SystemEvent::HomeCoreStarted));
+    }
+
+    #[tokio::test]
+    async fn fire_domain_reaches_subscriber() {
+        let bus = EventBus::new();
+        let mut rx = bus.subscribe_domain();
+        bus.fire_domain(DomainEvent::new(
+            "ruview_csi_frame",
+            serde_json::json!({"frame_id": 42}),
+            Context::new(),
+        ));
+        let event = rx.recv().await.unwrap();
+        assert_eq!(event.event_type, "ruview_csi_frame");
+        assert_eq!(event.event_data["frame_id"], 42);
+    }
+}
diff --git a/v2/crates/homecore/src/entity.rs b/v2/crates/homecore/src/entity.rs
new file mode 100644
index 00000000..e3308bb4
--- /dev/null
+++ b/v2/crates/homecore/src/entity.rs
@@ -0,0 +1,238 @@
+//! Entity ID newtype + immutable state snapshot type.
+//!
+//! Mirrors `homeassistant/core.py` `State` and the `entity_id` string
+//! validation that every public HA call performs.
+//!
+//! ## EntityId validation (ADR-127 §2.1 + Q1)
+//!
+//! HA accepts unicode entity IDs since 2024.3. HOMECORE P1 accepts the
+//! ASCII subset `[a-z0-9_]+\.[a-z0-9_]+` and rejects everything else
+//! with a clear error. Unicode acceptance is deferred to P2 once the
+//! Q1 strictness decision is made (see ADR-127 §8).
+
+use std::fmt;
+use std::sync::Arc;
+
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Deserializer, Serialize, Serializer};
+use thiserror::Error;
+
+use crate::event::Context;
+
+/// Validated `domain.name` entity identifier.
+///
+/// Construct via [`EntityId::parse`] or [`EntityId::new`]; both validate
+/// against the format `[a-z0-9_]+\.[a-z0-9_]+`. Custom `Serialize` /
+/// `Deserialize` round-trips as a plain JSON string (matching HA's wire
+/// format) and re-validates on deserialize so invalid IDs from disk
+/// fail at load time rather than at first use.
+#[derive(Clone, Eq, PartialEq, Hash)]
+pub struct EntityId(Arc<str>);
+
+impl Serialize for EntityId {
+    fn serialize<S: Serializer>(&self, ser: S) -> Result<S::Ok, S::Error> {
+        ser.serialize_str(&self.0)
+    }
+}
+
+impl<'de> Deserialize<'de> for EntityId {
+    fn deserialize<D: Deserializer<'de>>(de: D) -> Result<Self, D::Error> {
+        let s = String::deserialize(de)?;
+        EntityId::parse(s).map_err(serde::de::Error::custom)
+    }
+}
+
+impl EntityId {
+    /// Validates and constructs an `EntityId`. Returns
+    /// [`EntityIdError`] if the input is not `domain.name` shape with
+    /// ASCII lowercase / digits / underscore in each segment.
+    pub fn parse(s: impl Into<String>) -> Result<Self, EntityIdError> {
+        let s: String = s.into();
+        let (domain, name) = s
+            .split_once('.')
+            .ok_or_else(|| EntityIdError::MissingDot(s.clone()))?;
+        if domain.is_empty() {
+            return Err(EntityIdError::EmptyDomain(s));
+        }
+        if name.is_empty() {
+            return Err(EntityIdError::EmptyName(s));
+        }
+        for ch in domain.chars().chain(name.chars()) {
+            if !(ch.is_ascii_lowercase() || ch.is_ascii_digit() || ch == '_') {
+                return Err(EntityIdError::InvalidChar { entity_id: s, ch });
+            }
+        }
+        Ok(Self(Arc::from(s)))
+    }
+
+    /// Same as [`Self::parse`] but takes a `&str` and returns
+    /// `Result<&'static EntityId, ...>` for constant entity IDs known
+    /// at compile time. Used by ADR-128 plugins to register fixed-name
+    /// services like `homeassistant.restart`.
+    pub fn new(s: &str) -> Result<Self, EntityIdError> {
+        Self::parse(s.to_owned())
+    }
+
+    /// Returns the `domain` part (everything before the first `.`).
+    pub fn domain(&self) -> &str {
+        self.0.split_once('.').map(|(d, _)| d).unwrap_or(&self.0)
+    }
+
+    /// Returns the `name` part (everything after the first `.`).
+    pub fn name(&self) -> &str {
+        self.0.split_once('.').map(|(_, n)| n).unwrap_or("")
+    }
+
+    /// Underlying string view.
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+}
+
+impl fmt::Debug for EntityId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "EntityId({})", self.0)
+    }
+}
+
+impl fmt::Display for EntityId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&self.0)
+    }
+}
+
+#[derive(Error, Debug, Clone, Eq, PartialEq)]
+pub enum EntityIdError {
+    #[error("entity_id {0:?} is missing the required '.' between domain and name")]
+    MissingDot(String),
+    #[error("entity_id {0:?} has an empty domain segment")]
+    EmptyDomain(String),
+    #[error("entity_id {0:?} has an empty name segment")]
+    EmptyName(String),
+    #[error("entity_id {entity_id:?} contains invalid character {ch:?} — only [a-z0-9_] allowed (HA-compat ASCII subset; see ADR-127 §Q1)")]
+    InvalidChar { entity_id: String, ch: char },
+}
+
+/// Immutable state snapshot for one entity at one moment in time.
+///
+/// Mirrors `homeassistant.core.State`. Reader-cloneable via `Arc<State>`;
+/// writers atomically replace the entry in the `DashMap` so observers
+/// never see a partial mutation.
+#[derive(Clone, Debug, Serialize, Deserialize)]
+pub struct State {
+    pub entity_id: EntityId,
+    pub state: String,
+    /// Attribute bag — accepts whatever JSON the integration emits.
+    /// Mirrors HA's `Dict[str, Any]` attribute model.
+    pub attributes: serde_json::Value,
+    /// When the `state` field last changed value. Only bumped if the
+    /// new state string differs from the old; attribute-only updates
+    /// preserve this timestamp.
+    pub last_changed: DateTime<Utc>,
+    /// When this snapshot was written. Bumped on every `set` call,
+    /// including attribute-only updates.
+    pub last_updated: DateTime<Utc>,
+    /// Causality context — links state changes to the user / automation
+    /// / service call that originated them. Mirrors HA's `Context`.
+    pub context: Context,
+}
+
+impl State {
+    /// Construct a fresh state snapshot at `now`.
+    pub fn new(
+        entity_id: EntityId,
+        state: impl Into<String>,
+        attributes: serde_json::Value,
+        context: Context,
+    ) -> Self {
+        let now = Utc::now();
+        Self {
+            entity_id,
+            state: state.into(),
+            attributes,
+            last_changed: now,
+            last_updated: now,
+            context,
+        }
+    }
+
+    /// Construct the next state snapshot. If the new `state` string
+    /// equals the prior `state`, `last_changed` is preserved.
+    pub fn next(
+        &self,
+        new_state: impl Into<String>,
+        new_attributes: serde_json::Value,
+        context: Context,
+    ) -> Self {
+        let new_state = new_state.into();
+        let now = Utc::now();
+        let last_changed = if new_state == self.state {
+            self.last_changed
+        } else {
+            now
+        };
+        Self {
+            entity_id: self.entity_id.clone(),
+            state: new_state,
+            attributes: new_attributes,
+            last_changed,
+            last_updated: now,
+            context,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn entity_id_parses_valid() {
+        let e = EntityId::parse("light.living_room").unwrap();
+        assert_eq!(e.domain(), "light");
+        assert_eq!(e.name(), "living_room");
+        assert_eq!(e.as_str(), "light.living_room");
+    }
+
+    #[test]
+    fn entity_id_rejects_missing_dot() {
+        assert!(matches!(
+            EntityId::parse("light_living_room"),
+            Err(EntityIdError::MissingDot(_))
+        ));
+    }
+
+    #[test]
+    fn entity_id_rejects_uppercase() {
+        let err = EntityId::parse("light.LivingRoom").unwrap_err();
+        match err {
+            EntityIdError::InvalidChar { ch, .. } => assert_eq!(ch, 'L'),
+            other => panic!("expected InvalidChar, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn entity_id_rejects_unicode() {
+        // ADR-127 §Q1 — P1 is strict ASCII. Unicode acceptance deferred.
+        assert!(EntityId::parse("light.küche").is_err());
+    }
+
+    #[test]
+    fn state_next_preserves_last_changed_when_state_unchanged() {
+        let id = EntityId::parse("sensor.temp").unwrap();
+        let s1 = State::new(id.clone(), "20.0", serde_json::json!({}), Context::default());
+        std::thread::sleep(std::time::Duration::from_millis(2));
+        let s2 = s1.next("20.0", serde_json::json!({"updated": true}), Context::default());
+        assert_eq!(s1.last_changed, s2.last_changed);
+        assert!(s2.last_updated > s1.last_updated);
+    }
+
+    #[test]
+    fn state_next_bumps_last_changed_when_state_changes() {
+        let id = EntityId::parse("sensor.temp").unwrap();
+        let s1 = State::new(id, "20.0", serde_json::json!({}), Context::default());
+        std::thread::sleep(std::time::Duration::from_millis(2));
+        let s2 = s1.next("21.0", serde_json::json!({}), Context::default());
+        assert!(s2.last_changed > s1.last_changed);
+    }
+}
diff --git a/v2/crates/homecore/src/event.rs b/v2/crates/homecore/src/event.rs
new file mode 100644
index 00000000..7856a1be
--- /dev/null
+++ b/v2/crates/homecore/src/event.rs
@@ -0,0 +1,163 @@
+//! Typed system events + untyped domain events + Context.
+//!
+//! Mirrors `homeassistant.core.EventBus` + `homeassistant.const.EVENT_*`
+//! constants. ADR-127 §2.2 splits HA's single dict-typed event channel
+//! into two: a typed system channel (zero-allocation read path) and a
+//! json-blob domain channel (for arbitrary integration-fired events).
+
+use std::sync::Arc;
+
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+use uuid::Uuid;
+
+use crate::entity::{EntityId, State};
+
+/// Well-known HA event-type string constants.
+///
+/// Mirrors `homeassistant/const.py` `EVENT_*` constants. Used by
+/// integrations that fire untyped [`DomainEvent`]s.
+#[non_exhaustive]
+pub struct EventType;
+
+impl EventType {
+    pub const STATE_CHANGED: &'static str = "state_changed";
+    pub const SERVICE_REGISTERED: &'static str = "service_registered";
+    pub const SERVICE_REMOVED: &'static str = "service_removed";
+    pub const CALL_SERVICE: &'static str = "call_service";
+    pub const COMPONENT_LOADED: &'static str = "component_loaded";
+    pub const PLATFORM_DISCOVERED: &'static str = "platform_discovered";
+    pub const HOMEASSISTANT_START: &'static str = "homeassistant_start";
+    pub const HOMEASSISTANT_STARTED: &'static str = "homeassistant_started";
+    pub const HOMEASSISTANT_STOP: &'static str = "homeassistant_stop";
+    pub const HOMEASSISTANT_FINAL_WRITE: &'static str = "homeassistant_final_write";
+    pub const HOMEASSISTANT_CLOSE: &'static str = "homeassistant_close";
+}
+
+/// Causality context for a state change or service call.
+///
+/// Mirrors `homeassistant.core.Context`. Used by automations to detect
+/// loops ("don't re-fire on a state change my own automation caused")
+/// and by the recorder (ADR-132) to attribute changes to users.
+#[derive(Clone, Debug, Serialize, Deserialize, Eq, PartialEq)]
+pub struct Context {
+    pub id: Uuid,
+    pub user_id: Option<String>,
+    pub parent_id: Option<Uuid>,
+}
+
+impl Context {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    pub fn with_user(user_id: impl Into<String>) -> Self {
+        Self {
+            id: Uuid::new_v4(),
+            user_id: Some(user_id.into()),
+            parent_id: None,
+        }
+    }
+
+    pub fn child_of(parent: &Context) -> Self {
+        Self {
+            id: Uuid::new_v4(),
+            user_id: parent.user_id.clone(),
+            parent_id: Some(parent.id),
+        }
+    }
+}
+
+impl Default for Context {
+    fn default() -> Self {
+        Self {
+            id: Uuid::new_v4(),
+            user_id: None,
+            parent_id: None,
+        }
+    }
+}
+
+/// Typed enum of system events. Subscribers that only care about a
+/// specific shape (the recorder, the websocket subscriber) can match on
+/// the variant without going through `serde_json::Value`.
+#[derive(Clone, Debug)]
+pub enum SystemEvent {
+    StateChanged(StateChangedEvent),
+    ServiceRegistered { domain: String, service: String },
+    ServiceRemoved { domain: String, service: String },
+    ComponentLoaded { component: String },
+    HomeCoreStart,
+    HomeCoreStarted,
+    HomeCoreStop,
+}
+
+/// State-change event payload. Carries the old and new snapshots so a
+/// subscriber doesn't need to read the state machine again to learn
+/// what changed.
+///
+/// Mirrors HA's event_data `{ entity_id, old_state, new_state }`.
+#[derive(Clone, Debug)]
+pub struct StateChangedEvent {
+    pub entity_id: EntityId,
+    pub old_state: Option<Arc<State>>,
+    pub new_state: Option<Arc<State>>,
+    pub fired_at: DateTime<Utc>,
+}
+
+/// Untyped event fired by integrations. Mirrors HA's
+/// `EventBus.async_fire(event_type, event_data)`.
+#[derive(Clone, Debug)]
+pub struct DomainEvent {
+    pub event_type: String,
+    pub event_data: serde_json::Value,
+    pub origin: EventOrigin,
+    pub context: Context,
+    pub fired_at: DateTime<Utc>,
+}
+
+/// Where an event originated. Mirrors HA's `EventOrigin` enum (`local`
+/// vs `remote`).
+#[derive(Clone, Debug, Copy, Eq, PartialEq, Serialize, Deserialize)]
+pub enum EventOrigin {
+    Local,
+    Remote,
+}
+
+impl DomainEvent {
+    pub fn new(
+        event_type: impl Into<String>,
+        event_data: serde_json::Value,
+        context: Context,
+    ) -> Self {
+        Self {
+            event_type: event_type.into(),
+            event_data,
+            origin: EventOrigin::Local,
+            context,
+            fired_at: Utc::now(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn context_child_inherits_user_id() {
+        let parent = Context::with_user("alice");
+        let child = Context::child_of(&parent);
+        assert_eq!(child.user_id.as_deref(), Some("alice"));
+        assert_eq!(child.parent_id, Some(parent.id));
+        assert_ne!(child.id, parent.id);
+    }
+
+    #[test]
+    fn event_type_constants_match_ha_names() {
+        // These string values are wire-format with HA — must match
+        // exactly so ADR-130 can serve a wire-compat WebSocket API.
+        assert_eq!(EventType::STATE_CHANGED, "state_changed");
+        assert_eq!(EventType::HOMEASSISTANT_START, "homeassistant_start");
+    }
+}
diff --git a/v2/crates/homecore/src/homecore.rs b/v2/crates/homecore/src/homecore.rs
new file mode 100644
index 00000000..c71a8a03
--- /dev/null
+++ b/v2/crates/homecore/src/homecore.rs
@@ -0,0 +1,75 @@
+//! `HomeCore` runtime coordinator. Mirrors `homeassistant.core.HomeAssistant`.
+//!
+//! Cheap to clone — all internals are `Arc`-shared so tasks can each
+//! hold their own `HomeCore` handle without coordination overhead.
+
+use std::sync::Arc;
+
+use crate::bus::EventBus;
+use crate::registry::EntityRegistry;
+use crate::service::ServiceRegistry;
+use crate::state::StateMachine;
+
+#[derive(Clone)]
+pub struct HomeCore {
+    inner: Arc<HomeCoreInner>,
+}
+
+struct HomeCoreInner {
+    pub bus: EventBus,
+    pub states: StateMachine,
+    pub services: ServiceRegistry,
+    pub entities: EntityRegistry,
+}
+
+impl HomeCore {
+    pub fn new() -> Self {
+        Self {
+            inner: Arc::new(HomeCoreInner {
+                bus: EventBus::new(),
+                states: StateMachine::new(),
+                services: ServiceRegistry::new(),
+                entities: EntityRegistry::new(),
+            }),
+        }
+    }
+
+    pub fn bus(&self) -> &EventBus {
+        &self.inner.bus
+    }
+
+    pub fn states(&self) -> &StateMachine {
+        &self.inner.states
+    }
+
+    pub fn services(&self) -> &ServiceRegistry {
+        &self.inner.services
+    }
+
+    pub fn entities(&self) -> &EntityRegistry {
+        &self.inner.entities
+    }
+}
+
+impl Default for HomeCore {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::entity::EntityId;
+    use crate::event::Context;
+
+    #[tokio::test]
+    async fn end_to_end_set_then_get() {
+        let hc = HomeCore::new();
+        let id = EntityId::parse("light.kitchen").unwrap();
+        hc.states().set(id.clone(), "on", serde_json::json!({"brightness": 200}), Context::new());
+        let snap = hc.states().get(&id).unwrap();
+        assert_eq!(snap.state, "on");
+        assert_eq!(snap.attributes["brightness"], 200);
+    }
+}
diff --git a/v2/crates/homecore/src/lib.rs b/v2/crates/homecore/src/lib.rs
new file mode 100644
index 00000000..ef0b7e39
--- /dev/null
+++ b/v2/crates/homecore/src/lib.rs
@@ -0,0 +1,59 @@
+//! HOMECORE — Rust port of `homeassistant/core.py`.
+//!
+//! Implements [ADR-127](../../docs/adr/ADR-127-homecore-state-machine-rust.md):
+//! the state machine, event bus, service registry, and entity registry that
+//! every other HOMECORE module depends on.
+//!
+//! ## Layout (P1 scaffold)
+//!
+//! - [`entity`] — `EntityId` newtype + validation; `State` snapshot type
+//! - [`event`] — typed `SystemEvent` + untyped `DomainEvent` + `Context`
+//! - [`state`] — `StateMachine`: DashMap-backed concurrent state store
+//! - [`bus`] — `EventBus`: tokio broadcast wiring for system + domain events
+//! - [`service`] — `ServiceRegistry` (stub; full mpsc dispatch lands in P2)
+//! - [`registry`] — `EntityRegistry` (in-memory P1; persistence lands in P2)
+//! - [`homecore`] — `HomeCore` runtime coordinator: holds bus + states + services
+//!
+//! ## Threading model
+//!
+//! HOMECORE is multi-threaded — concurrent reads from any number of tasks
+//! return zero-copy `Arc<State>` clones. Writes are serialised per-entity
+//! by the DashMap shard lock but the global state machine itself is never
+//! locked. See ADR-127 §2.1.
+//!
+//! ## What's NOT here yet (deferred to P2+)
+//!
+//! - Persistence of entity registry to `.homecore/storage/core.entity_registry`
+//! - Schema validation (`schemas` module from §3 stub)
+//! - Service handler mpsc dispatch (`service::ServiceRegistry::call`)
+//! - Device registry (mirror of HA's `core.device_registry`)
+//! - Witness chain integration (ADR-028)
+//!
+//! Each is marked `// TODO P2:` at the relevant call site.
+
+pub mod entity;
+pub mod event;
+pub mod state;
+pub mod bus;
+pub mod service;
+pub mod registry;
+
+mod homecore;
+
+pub use homecore::HomeCore;
+
+pub use entity::{EntityId, EntityIdError, State};
+pub use event::{Context, DomainEvent, EventType, StateChangedEvent, SystemEvent};
+pub use state::StateMachine;
+pub use bus::EventBus;
+pub use service::{ServiceCall, ServiceError, ServiceName, ServiceRegistry};
+pub use registry::{EntityCategory, EntityEntry, EntityRegistry};
+
+/// HOMECORE protocol/data-model version. Bumped when the public surface
+/// or on-disk persistence schema changes in a backwards-incompatible way.
+/// Mirrors HA's `core.entity_registry` schema version (currently 13).
+pub const HOMECORE_VERSION: u32 = 1;
+
+/// Compile-time identifier for the HOMECORE build. Wired in by `vergen`
+/// or git SHA in a later phase; constant for now.
+pub const HOMECORE_BUILD_TAG: &str = env!("CARGO_PKG_VERSION");
diff --git a/v2/crates/homecore/src/registry.rs b/v2/crates/homecore/src/registry.rs
new file mode 100644
index 00000000..2e637956
--- /dev/null
+++ b/v2/crates/homecore/src/registry.rs
@@ -0,0 +1,130 @@
+//! In-memory entity registry (P1). Persistence to
+//! `.homecore/storage/core.entity_registry` lands in P2.
+//!
+//! Schema fields mirror HA `core.entity_registry` v13 per ADR-127 §2.4.
+
+use std::collections::HashMap;
+use std::sync::Arc;
+
+use serde::{Deserialize, Serialize};
+use tokio::sync::RwLock;
+
+use crate::entity::EntityId;
+
+/// Entity category enum. Mirrors HA `homeassistant.helpers.entity.EntityCategory`.
+#[derive(Clone, Debug, Eq, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum EntityCategory {
+    Config,
+    Diagnostic,
+}
+
+/// Source that disabled an entity. Mirrors HA `disabled_by` enum.
+#[derive(Clone, Debug, Eq, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum DisabledBy {
+    User,
+    Integration,
+    ConfigEntry,
+    Device,
+}
+
+#[derive(Clone, Debug, Serialize, Deserialize)]
+pub struct EntityEntry {
+    pub entity_id: EntityId,
+    pub unique_id: Option<String>,
+    pub platform: String,
+    /// User-set display name. None means "use the entity's default name".
+    pub name: Option<String>,
+    pub disabled_by: Option<DisabledBy>,
+    pub area_id: Option<String>,
+    pub device_id: Option<String>,
+    pub entity_category: Option<EntityCategory>,
+    pub config_entry_id: Option<String>,
+}
+
+#[derive(Clone)]
+pub struct EntityRegistry {
+    entries: Arc<RwLock<HashMap<EntityId, EntityEntry>>>,
+}
+
+impl EntityRegistry {
+    pub fn new() -> Self {
+        Self {
+            entries: Arc::new(RwLock::new(HashMap::new())),
+        }
+    }
+
+    pub async fn register(&self, entry: EntityEntry) {
+        self.entries.write().await.insert(entry.entity_id.clone(), entry);
+    }
+
+    pub async fn get(&self, entity_id: &EntityId) -> Option<EntityEntry> {
+        self.entries.read().await.get(entity_id).cloned()
+    }
+
+    pub async fn remove(&self, entity_id: &EntityId) -> Option<EntityEntry> {
+        self.entries.write().await.remove(entity_id)
+    }
+
+    pub async fn all(&self) -> Vec<EntityEntry> {
+        self.entries.read().await.values().cloned().collect()
+    }
+
+    pub async fn len(&self) -> usize {
+        self.entries.read().await.len()
+    }
+}
+
+impl Default for EntityRegistry {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn register_and_read() {
+        let reg = EntityRegistry::new();
+        let id = EntityId::parse("light.kitchen").unwrap();
+        reg.register(EntityEntry {
+            entity_id: id.clone(),
+            unique_id: Some("hue_lamp_42".into()),
+            platform: "hue".into(),
+            name: Some("Kitchen lamp".into()),
+            disabled_by: None,
+            area_id: Some("kitchen".into()),
+            device_id: None,
+            entity_category: None,
+            config_entry_id: None,
+        })
+        .await;
+        let got = reg.get(&id).await.unwrap();
+        assert_eq!(got.platform, "hue");
+        assert_eq!(got.name.as_deref(), Some("Kitchen lamp"));
+    }
+
+    #[tokio::test]
+    async fn disabled_by_round_trips_via_serde() {
+        let entry = EntityEntry {
+            entity_id: EntityId::parse("sensor.x").unwrap(),
+            unique_id: None,
+            platform: "test".into(),
+            name: None,
+            disabled_by: Some(DisabledBy::Integration),
+            area_id: None,
+            device_id: None,
+            entity_category: Some(EntityCategory::Diagnostic),
+            config_entry_id: None,
+        };
+        let json = serde_json::to_string(&entry).unwrap();
+        // HA wire format uses snake_case for the disabled_by enum.
+        assert!(json.contains("\"disabled_by\":\"integration\""));
+        assert!(json.contains("\"entity_category\":\"diagnostic\""));
+        let back: EntityEntry = serde_json::from_str(&json).unwrap();
+        assert_eq!(back.disabled_by, Some(DisabledBy::Integration));
+    }
+}
diff --git a/v2/crates/homecore/src/service.rs b/v2/crates/homecore/src/service.rs
new file mode 100644
index 00000000..68db2d5c
--- /dev/null
+++ b/v2/crates/homecore/src/service.rs
@@ -0,0 +1,170 @@
+//! Service registry stub.
+//!
+//! Mirrors `homeassistant.core.ServiceRegistry`. P1 ships the public
+//! surface + a simple direct-dispatch `call` so downstream ADRs can
+//! depend on it; ADR-127 P2 replaces direct dispatch with the
+//! mpsc-router pattern described in §2.3.
+
+use std::collections::HashMap;
+use std::future::Future;
+use std::pin::Pin;
+use std::sync::Arc;
+
+use async_trait::async_trait;
+use serde::{Deserialize, Serialize};
+use thiserror::Error;
+use tokio::sync::RwLock;
+
+use crate::event::Context;
+
+/// Service name within a domain. e.g. `light.turn_on` → domain
+/// `"light"`, service `"turn_on"`.
+#[derive(Clone, Debug, Eq, PartialEq, Hash, Serialize, Deserialize)]
+pub struct ServiceName {
+    pub domain: String,
+    pub service: String,
+}
+
+impl ServiceName {
+    pub fn new(domain: impl Into<String>, service: impl Into<String>) -> Self {
+        Self {
+            domain: domain.into(),
+            service: service.into(),
+        }
+    }
+}
+
+/// Inbound service-call payload. Mirrors HA's `service_data` dict
+/// plus the originating `Context`.
+#[derive(Clone, Debug)]
+pub struct ServiceCall {
+    pub name: ServiceName,
+    pub data: serde_json::Value,
+    pub context: Context,
+}
+
+#[derive(Error, Debug)]
+pub enum ServiceError {
+    #[error("service not registered: {domain}.{service}")]
+    NotRegistered { domain: String, service: String },
+    #[error("service handler returned error: {0}")]
+    HandlerFailed(String),
+}
+
+/// Handler trait. Integration code implements this and registers via
+/// [`ServiceRegistry::register`]. P2 will add schema validation via
+/// `serde` `Deserialize<'_>`.
+#[async_trait]
+pub trait ServiceHandler: Send + Sync + 'static {
+    async fn call(&self, call: ServiceCall) -> Result<serde_json::Value, ServiceError>;
+}
+
+/// Direct closure adapter so simple handlers don't need a struct.
+pub struct FnHandler<F>(pub F);
+
+#[async_trait]
+impl<F, Fut> ServiceHandler for FnHandler<F>
+where
+    F: Fn(ServiceCall) -> Fut + Send + Sync + 'static,
+    Fut: Future<Output = Result<serde_json::Value, ServiceError>> + Send + 'static,
+{
+    async fn call(&self, call: ServiceCall) -> Result<serde_json::Value, ServiceError> {
+        (self.0)(call).await
+    }
+}
+
+#[derive(Clone)]
+pub struct ServiceRegistry {
+    handlers: Arc<RwLock<HashMap<ServiceName, Arc<dyn ServiceHandler>>>>,
+}
+
+impl ServiceRegistry {
+    pub fn new() -> Self {
+        Self {
+            handlers: Arc::new(RwLock::new(HashMap::new())),
+        }
+    }
+
+    pub async fn register<H: ServiceHandler>(&self, name: ServiceName, handler: H) {
+        self.handlers.write().await.insert(name, Arc::new(handler));
+    }
+
+    pub async fn remove(&self, name: &ServiceName) {
+        self.handlers.write().await.remove(name);
+    }
+
+    pub async fn has(&self, name: &ServiceName) -> bool {
+        self.handlers.read().await.contains_key(name)
+    }
+
+    /// Call a service. P1 direct dispatch; P2 routes through the
+    /// event bus per ADR-127 §2.3.
+    pub async fn call(&self, call: ServiceCall) -> Result<serde_json::Value, ServiceError> {
+        let handler = {
+            let guard = self.handlers.read().await;
+            guard.get(&call.name).cloned()
+        };
+        match handler {
+            Some(h) => h.call(call).await,
+            None => Err(ServiceError::NotRegistered {
+                domain: call.name.domain.clone(),
+                service: call.name.service.clone(),
+            }),
+        }
+    }
+
+    pub async fn registered_services(&self) -> Vec<ServiceName> {
+        self.handlers.read().await.keys().cloned().collect()
+    }
+}
+
+impl Default for ServiceRegistry {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+// Suppress unused-import warning when no consumer of Pin/Box uses them yet
+#[allow(dead_code)]
+type _UnusedFutureType = Pin<Box<dyn Future<Output = ()> + Send>>;
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn register_and_call_returns_handler_value() {
+        let reg = ServiceRegistry::new();
+        reg.register(
+            ServiceName::new("light", "turn_on"),
+            FnHandler(|call: ServiceCall| async move {
+                Ok(serde_json::json!({"called_with": call.data}))
+            }),
+        )
+        .await;
+
+        let resp = reg
+            .call(ServiceCall {
+                name: ServiceName::new("light", "turn_on"),
+                data: serde_json::json!({"brightness": 200}),
+                context: Context::new(),
+            })
+            .await
+            .unwrap();
+        assert_eq!(resp["called_with"]["brightness"], 200);
+    }
+
+    #[tokio::test]
+    async fn unregistered_service_returns_error() {
+        let reg = ServiceRegistry::new();
+        let err = reg
+            .call(ServiceCall {
+                name: ServiceName::new("light", "turn_on"),
+                data: serde_json::json!({}),
+                context: Context::new(),
+            })
+            .await
+            .unwrap_err();
+        assert!(matches!(err, ServiceError::NotRegistered { .. }));
+    }
+}
diff --git a/v2/crates/homecore/src/state.rs b/v2/crates/homecore/src/state.rs
new file mode 100644
index 00000000..5563af86
--- /dev/null
+++ b/v2/crates/homecore/src/state.rs
@@ -0,0 +1,221 @@
+//! Concurrent state machine — the heart of HOMECORE.
+//!
+//! Mirrors `homeassistant.core.StateMachine`. Differences from HA per
+//! ADR-127 §2.1:
+//!
+//! - DashMap shard-locked instead of one asyncio.Lock for the whole map
+//! - Writers atomically replace `Arc<State>` entries; readers get
+//!   zero-copy clones
+//! - State changes fan out via a tokio broadcast channel (capacity
+//!   4,096); slow subscribers get `Lagged` and must re-sync from the
+//!   current map
+//!
+//! ## NOT in P1 (deferred to P2+)
+//!
+//! - `async_set_internal` schema validation
+//! - Bulk delete of an entire domain (`async_remove_domain`)
+//! - Restore-state on startup from the recorder (ADR-132)
+
+use std::sync::Arc;
+
+use chrono::Utc;
+use dashmap::DashMap;
+use tokio::sync::broadcast;
+
+use crate::entity::{EntityId, State};
+use crate::event::{Context, StateChangedEvent};
+
+/// Broadcast channel capacity for state-changed events. 4,096 events
+/// at 20 Hz per entity covers ~3 minutes of backlog for a single hot
+/// entity. Slow subscribers must re-sync from the current map.
+pub const STATE_CHANGED_CHANNEL_CAPACITY: usize = 4096;
+
+/// The state machine. Cheap to clone (one `Arc`) — pass copies to as
+/// many tasks as you like.
+#[derive(Clone)]
+pub struct StateMachine {
+    inner: Arc<StateMachineInner>,
+}
+
+struct StateMachineInner {
+    states: DashMap<EntityId, Arc<State>>,
+    tx: broadcast::Sender<StateChangedEvent>,
+}
+
+impl StateMachine {
+    pub fn new() -> Self {
+        let (tx, _) = broadcast::channel(STATE_CHANGED_CHANNEL_CAPACITY);
+        Self {
+            inner: Arc::new(StateMachineInner {
+                states: DashMap::with_capacity(256),
+                tx,
+            }),
+        }
+    }
+
+    /// Subscribe to state-changed events. Each subscriber gets an
+    /// independent receiver; capacity is shared. Falling behind by
+    /// 4,096 events yields `RecvError::Lagged(n)`.
+    pub fn subscribe(&self) -> broadcast::Receiver<StateChangedEvent> {
+        self.inner.tx.subscribe()
+    }
+
+    /// Read a state. Returns `None` if the entity is unknown.
+    /// Zero-copy: caller gets an `Arc<State>` clone.
+    pub fn get(&self, entity_id: &EntityId) -> Option<Arc<State>> {
+        self.inner.states.get(entity_id).map(|s| Arc::clone(&s))
+    }
+
+    /// Write a state. Fires a `state_changed` broadcast even on the
+    /// first write (old_state = None). HA semantics: only fires if the
+    /// state string OR attributes changed; pure no-op writes are
+    /// suppressed.
+    ///
+    /// Returns the new state snapshot.
+    pub fn set(
+        &self,
+        entity_id: EntityId,
+        new_state: impl Into<String>,
+        attributes: serde_json::Value,
+        context: Context,
+    ) -> Arc<State> {
+        let new_state_str = new_state.into();
+        let old = self.inner.states.get(&entity_id).map(|r| Arc::clone(&*r));
+
+        let next = match &old {
+            Some(prev) => Arc::new(prev.next(new_state_str.clone(), attributes.clone(), context)),
+            None => Arc::new(State::new(entity_id.clone(), new_state_str.clone(), attributes.clone(), context)),
+        };
+
+        // HA suppresses no-op writes (same state + same attributes).
+        // We follow the same rule to keep the broadcast channel quiet.
+        let is_noop = match &old {
+            Some(prev) => prev.state == new_state_str && prev.attributes == attributes,
+            None => false,
+        };
+
+        self.inner.states.insert(entity_id.clone(), Arc::clone(&next));
+
+        if !is_noop {
+            let event = StateChangedEvent {
+                entity_id,
+                old_state: old,
+                new_state: Some(Arc::clone(&next)),
+                fired_at: Utc::now(),
+            };
+            // err = no receivers; that's fine, write still committed.
+            let _ = self.inner.tx.send(event);
+        }
+        next
+    }
+
+    /// Remove a state. Fires `state_changed` with `new_state = None`.
+    pub fn remove(&self, entity_id: &EntityId) -> Option<Arc<State>> {
+        let removed = self.inner.states.remove(entity_id).map(|(_, s)| s);
+        if let Some(old) = &removed {
+            let event = StateChangedEvent {
+                entity_id: entity_id.clone(),
+                old_state: Some(Arc::clone(old)),
+                new_state: None,
+                fired_at: Utc::now(),
+            };
+            let _ = self.inner.tx.send(event);
+        }
+        removed
+    }
+
+    /// Snapshot all current states. Allocates a new Vec — useful for
+    /// the REST GET /api/states path (ADR-130).
+    pub fn all(&self) -> Vec<Arc<State>> {
+        self.inner.states.iter().map(|r| Arc::clone(r.value())).collect()
+    }
+
+    /// Snapshot all states whose entity_id matches a domain prefix.
+    /// Mirrors HA's `hass.states.async_all(domain)`.
+    pub fn all_by_domain(&self, domain: &str) -> Vec<Arc<State>> {
+        self.inner
+            .states
+            .iter()
+            .filter(|r| r.key().domain() == domain)
+            .map(|r| Arc::clone(r.value()))
+            .collect()
+    }
+
+    /// Number of entities currently tracked.
+    pub fn len(&self) -> usize {
+        self.inner.states.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.inner.states.len() == 0
+    }
+}
+
+impl Default for StateMachine {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn id(s: &str) -> EntityId {
+        EntityId::parse(s).unwrap()
+    }
+
+    #[tokio::test]
+    async fn set_writes_and_fires() {
+        let sm = StateMachine::new();
+        let mut rx = sm.subscribe();
+        sm.set(id("light.kitchen"), "on", serde_json::json!({"brightness": 200}), Context::new());
+        let evt = rx.recv().await.unwrap();
+        assert_eq!(evt.entity_id.as_str(), "light.kitchen");
+        assert!(evt.old_state.is_none());
+        assert_eq!(evt.new_state.as_ref().unwrap().state, "on");
+    }
+
+    #[tokio::test]
+    async fn noop_writes_are_suppressed() {
+        let sm = StateMachine::new();
+        sm.set(id("light.k"), "on", serde_json::json!({}), Context::new());
+        let mut rx = sm.subscribe();
+        // Same state + same attributes → no event.
+        sm.set(id("light.k"), "on", serde_json::json!({}), Context::new());
+        let try_recv = tokio::time::timeout(std::time::Duration::from_millis(50), rx.recv()).await;
+        assert!(try_recv.is_err(), "expected no event for no-op write");
+    }
+
+    #[tokio::test]
+    async fn attribute_only_change_fires_but_preserves_last_changed() {
+        let sm = StateMachine::new();
+        let s1 = sm.set(id("sensor.t"), "20", serde_json::json!({"unit": "C"}), Context::new());
+        tokio::time::sleep(std::time::Duration::from_millis(2)).await;
+        let s2 = sm.set(id("sensor.t"), "20", serde_json::json!({"unit": "F"}), Context::new());
+        assert_eq!(s1.last_changed, s2.last_changed);
+        assert!(s2.last_updated > s1.last_updated);
+    }
+
+    #[test]
+    fn all_by_domain_filters() {
+        let sm = StateMachine::new();
+        sm.set(id("light.a"), "on", serde_json::json!({}), Context::new());
+        sm.set(id("light.b"), "off", serde_json::json!({}), Context::new());
+        sm.set(id("sensor.t"), "20", serde_json::json!({}), Context::new());
+        assert_eq!(sm.all_by_domain("light").len(), 2);
+        assert_eq!(sm.all_by_domain("sensor").len(), 1);
+        assert_eq!(sm.all().len(), 3);
+    }
+
+    #[tokio::test]
+    async fn remove_fires_with_no_new_state() {
+        let sm = StateMachine::new();
+        sm.set(id("light.k"), "on", serde_json::json!({}), Context::new());
+        let mut rx = sm.subscribe();
+        sm.remove(&id("light.k"));
+        let evt = rx.recv().await.unwrap();
+        assert!(evt.new_state.is_none());
+        assert!(evt.old_state.is_some());
+    }
+}