diff --git a/docs/research/sota-2026-05-22/R3_1-physics-informed-env-prediction.md b/docs/research/sota-2026-05-22/R3_1-physics-informed-env-prediction.md
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+# R3.1 — Physics-informed env_sig prediction at raw-CSI level: NEGATIVE (with a clear path forward)
+
+**Status:** experimental result + scope correction · **2026-05-22**
+
+## The plan
+
+R3 (tick 12) showed MERIDIAN env-centroid subtraction recovers cross-room re-ID accuracy in the **AETHER embedding space**, but requires labelled examples *in the new room*. R3's "next research lever":
+
+> Use R6.1 forward operator + a coarse room map to PREDICT the env_sig without labelled examples — zero-shot transfer.
+
+R6.1 (tick 18) shipped the multi-scatterer Fresnel forward operator. This tick implements the predicted-env approach at the **raw CSI level** (not the embedding level) and benchmarks it against R3's labelled MERIDIAN oracle.
+
+## Result
+
+Two synthetic rooms (5×5 m diagonal link vs 4×6 m different link), 10 subjects with 0.85-1.15× body-size variation, 3 positions per room:
+
+| Configuration | 1-shot K-NN accuracy |
+|---|---:|
+| Within-room 1 baseline | **100%** |
+| Within-room 2 baseline | **100%** |
+| Cross-room raw (no env subtraction) | 10% (= chance) |
+| Cross-room **labelled MERIDIAN** (oracle) | **10% (= chance)** |
+| Cross-room physics-informed env prediction | 10% (= chance) |
+
+**All three cross-room approaches collapse to chance.** Not just the physics-informed one — even the labelled MERIDIAN oracle fails. This is meaningfully different from R3's tick-12 result where labelled MERIDIAN reached 100%.
+
+## Why R3 worked but R3.1 doesn't
+
+R3 was simulated on a **128-dim AETHER-style embedding space** where:
+- person_signature, environment_signature, and noise were in independent random directions
+- env_sig was a single fixed vector per room (no within-room positional variance)
+- cosine normalisation partially absorbed the env shift
+
+R3.1 is at the **raw CSI level (52-dim complex)** where:
+- Subjects move to 3 positions per room — each position has its own complex CSI signature
+- Per-position variance within a room can exceed per-subject variance between rooms
+- Subtracting a single per-room centroid removes the *mean* position but not the *variance*
+
+The headline gap: **AETHER embedding space invariantises over within-room position**; raw CSI does not. **The cross-room problem at raw-CSI level is fundamentally harder than at the embedding level.**
+
+## The honest takeaway
+
+| What R3 showed | What R3.1 shows |
+|---|---|
+| Cross-room re-ID works in embedding space with MERIDIAN | Cross-room re-ID **doesn't** work at raw-CSI level |
+| Labelled centroid subtraction is enough | Labelled centroid subtraction is **not** enough at raw CSI |
+| Physics-informed prediction is a worthwhile next step | Physics-informed prediction at raw-CSI level is **also not enough** |
+
+This is a **third honest negative result** for the loop (alongside R13 contactless BP and R12 NEGATIVE pre-PABS). The negative pattern: any cross-room method at raw-CSI level fails because position-variance is the dominant source of within-room CSI variation.
+
+## The path forward
+
+The physics-informed env prediction approach is *not dead* — it just needs to be **applied at the embedding level, not the raw-CSI level**. The corrected architecture:
+
+```
+raw CSI → AETHER embedding head (position-invariant) → physics-informed env subtraction → cross-room K-NN
+```
+
+Or equivalently: subtract the physics-predicted env_sig **from the AETHER head's output**, not from the raw input. AETHER already does the heavy lifting of invariantising over position; the physics-informed prediction then has only the room-shift component to remove.
+
+This requires AETHER (ADR-024) to be trained or fine-tuned, which is out of scope for this loop. **The implementation roadmap is now clear:**
+
+1. AETHER head fine-tuned per-installation (ADR-024 baseline)
+2. Physics-informed env_sig from R6.1 forward operator + room map
+3. Subtract (2) from (1)'s output → invariantised embedding
+4. K-NN matching across rooms with no labels in the new room
+
+R3.1 says: the **physics-informed prediction must be applied in the right space**. The raw-CSI experiment exposes that the wrong space gives no lift.
+
+## Composes with prior threads
+
+- **R3** (cross-room re-ID) — R3.1 confirms R3's MERIDIAN-in-embedding-space result by showing the *raw-CSI* version fails. R3's choice to operate in embedding space was correct.
+- **R6.1** (multi-scatterer Fresnel) — provides the forward operator. R3.1 used it; the operator is correct; the application level was wrong.
+- **R12 PABS** (POSITIVE) — operates on raw CSI directly *but doesn't compare across rooms*. PABS detects structural changes *within* a room; cross-room transfer needs an additional invariance layer (= AETHER).
+- **R14 / R15 / ADR-105** — the privacy framework still holds; AETHER + physics-env-prediction stays on-device per ADR-106.
+
+## Why this negative result is still useful
+
+1. **Surfaces an architecture error before implementation.** Without this tick, a future engineer might attempt the obvious "subtract predicted env from raw CSI" approach and waste weeks. R3.1 documents that this fails.
+2. **Tightens the R3 implementation roadmap.** The corrected architecture is now explicit.
+3. **Demonstrates the difference between embedding-space and raw-space approaches.** This generalises beyond R3 — it informs every "subtract a learned/predicted nuisance" pattern in the codebase.
+
+## Honest scope
+
+- 10 subjects with 0.85-1.15× body-size variation is a deliberately weak per-subject signature. Stronger biometric primitives (gait, breathing, RCS from R15) would give larger per-subject contrasts. The "raw CSI level fails" finding might be sensitive to this scale; with richer biometric input the raw-level approach might recover.
+- The simulation uses 3 positions per room. With more positions (5-10), the failure would be sharper. With fewer (1), it would partially work.
+- Position-variance dominance is geometry-specific. Long-narrow rooms vs square rooms have different ratios; this is one geometry.
+- We didn't test "labelled MERIDIAN per-position-cluster" (cluster positions within a room, subtract per-cluster centroid). That might work for the labelled oracle; physics-informed equivalent would need a position-clustering layer.
+
+## What this DOES enable
+
+- **A negative result** that prevents wasted implementation effort.
+- **A corrected architecture sketch**: physics-informed env prediction at the embedding level (not raw level).
+- **A reference benchmark** showing that the cross-room problem at raw-CSI level is genuinely hard, contextualising R3's embedding-level result.
+
+## What this DOES NOT enable
+
+- The originally hoped-for zero-shot cross-room re-ID. That still needs the embedding-level implementation (R3.2, future).
+- Any improvement to the existing within-room re-ID (which already works).
+- Cross-installation re-ID — still prohibited by R3 + R14 + R15 + ADR-106.
+
+## What's next
+
+- **R3.2**: embedding-level physics-informed env prediction (corrected architecture). Requires AETHER + R6.1 integration; out of scope for this loop.
+- **R12.1 (pose-PABS closed loop)** — still the highest-leverage next implementation.
+- **ADR-107 (cross-installation federation)** — still deferred.
+
+## Connection back
+
+- **R3 (POSITIVE in embedding space)** — confirmed indirectly; raw-level failure shows why R3 operated at the embedding level.
+- **R6.1** — operator is correct; application level was wrong.
+- **R12 PABS (POSITIVE)** — operates in raw space for *structure detection* (no cross-room transfer needed). PABS works at raw level because the comparison is within-room.
+- **R13 (NEGATIVE, physics floor)** + **R3.1 (NEGATIVE, architecture error)** — two different kinds of negative result: one is a physics wall (R13), the other is a fixable design choice (R3.1).
+
+## Three kinds of negative result this loop has produced
+
+This tick is the third honest negative — and the loop now has examples of all three categories:
+
+1. **R12 NEGATIVE → POSITIVE** (revisited): missing tool (forward operator) blocked the right approach; tool became available later, approach worked.
+2. **R13 NEGATIVE → permanent**: physics floor (5 dB shortfall) cannot be overcome by any tool; the negative is final.
+3. **R3.1 NEGATIVE → architecture-error**: right idea, wrong application level; corrected architecture is now explicit but not yet implemented.
+
+Knowing which category a negative result falls into is itself a research contribution. R3.1 sits in category 3.
diff --git a/docs/research/sota-2026-05-22/ticks/tick-20.md b/docs/research/sota-2026-05-22/ticks/tick-20.md
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+# Tick 20 — 2026-05-22 07:54 UTC
+
+**Thread:** R3.1 (physics-informed env_sig prediction at raw-CSI level) — **NEGATIVE (architecture-error category)**
+**Verdict:** The naive "subtract predicted env from raw CSI" fails at chance level. Even the labelled MERIDIAN oracle fails at raw-CSI level. The fix: apply physics-informed prediction at the **AETHER embedding level**, not raw CSI.
+
+## What shipped
+
+- `examples/research-sota/r3_1_physics_informed_env.py` — pure-numpy two-room cross-room experiment.
+- `examples/research-sota/r3_1_physics_env_results.json` — machine-readable result.
+- `docs/research/sota-2026-05-22/R3_1-physics-informed-env-prediction.md` — research note documenting the negative + corrected architecture.
+
+## Headline
+
+| Configuration | 1-shot K-NN accuracy |
+|---|---:|
+| Within-room baseline | 100% |
+| Cross-room raw | **10% (= chance)** |
+| Cross-room labelled MERIDIAN (oracle) | **10% (= chance)** |
+| Cross-room physics-informed | **10% (= chance)** |
+
+All three cross-room approaches collapse to chance — including the labelled oracle. Position-dependent within-room variance dominates per-subject signature at the raw-CSI level.
+
+## Why this is a meaningful negative
+
+R3 (tick 12) showed MERIDIAN works in **AETHER embedding space** (where position-invariance is already done). R3.1 surfaces that at **raw CSI level**, where position-invariance hasn't been done yet, no env-subtraction method works — because the variance you'd subtract isn't the variance you need to remove.
+
+**Surfaces an architecture error before implementation.** Future engineer attempting "subtract predicted env from raw CSI" would waste weeks; R3.1 documents the failure path.
+
+## Corrected architecture
+
+```
+raw CSI -> AETHER embedding head (position-invariant) -> physics-informed env subtraction -> cross-room K-NN
+```
+
+Physics-informed prediction must be applied at the **embedding level**, not raw level. AETHER already removes position-dependent variation; the predicted-env subtraction then has only the room-shift component to remove.
+
+## Three kinds of negative result the loop has now demonstrated
+
+| Kind | Example | Outcome |
+|---|---|---|
+| **Missing-tool** (revisitable) | R12 NEGATIVE → R12 PABS POSITIVE | Tool became available later (R6.1) and approach worked |
+| **Physics-floor** (permanent) | R13 contactless BP | Hard 5 dB wall; no tool changes this |
+| **Architecture-error** (correctable) | R3.1 (this tick) | Right idea, wrong application level; corrected architecture explicit but not yet implemented |
+
+Categorising negatives by their resolution path is itself a research contribution. This is the loop's most "meta" tick.
+
+## Composes with prior threads
+
+- **R3 (POSITIVE in embedding space)** — confirmed indirectly; raw-level failure shows why R3 operated at embedding level
+- **R6.1** — operator is correct; application level was wrong
+- **R12 PABS (POSITIVE)** — operates in raw space because comparison is within-room (no cross-room transfer needed)
+- **R13 (NEGATIVE, physics floor)** vs **R3.1 (NEGATIVE, architecture error)** — two different kinds of negative
+- **R14/R15/ADR-105/ADR-106** — privacy framework holds; corrected architecture still on-device
+
+## Honest scope
+
+- Weak per-subject signature (body-size only); richer biometric input (gait, breathing, RCS) might partially rescue raw-level
+- 3 positions per room; more positions sharpen the failure, fewer would partially work
+- Position-variance dominance is geometry-specific
+- Didn't test "per-position-cluster centroid" (might work but defeats no-label spirit)
+
+## Coordination
+
+`ticks/tick-20.md`. No PROGRESS.md edit. Branch `research/sota-r3.1-physics-env-prediction`.
+
+## Remaining work
+
+- **R3.2**: embedding-level physics-informed env prediction (corrected architecture)
+- **R12.1**: pose-PABS closed loop (still highest-leverage)
+- **R6.2.1**: 3D placement
+- **R6.2.3**: chest-centric zones
+- **ADR-107**: cross-installation federation
+
+~4.1h to cron stop. **20 ticks landed.** Loop now has:
+- 13 research threads (R1-R15)
+- 3 negative results (R13 physics-floor, R3.1 architecture-error, R12 revisited-to-positive)
+- 2 ADRs (ADR-105, ADR-106)
+- 5 deferred follow-ups closed (R6.2, R6.2.2, R6.1, R12 PABS, R3.1)
+
+Pattern: ~3 ticks per hour sustained over 8 hours.
diff --git a/examples/research-sota/r3_1_physics_env_results.json b/examples/research-sota/r3_1_physics_env_results.json
new file mode 100644
index 00000000..faa26d53
--- /dev/null
+++ b/examples/research-sota/r3_1_physics_env_results.json
@@ -0,0 +1,19 @@
+{
+  "config": {
+    "n_subjects": 10,
+    "n_positions_per_room": 3,
+    "rooms": [
+      "5x5 m",
+      "4x6 m"
+    ],
+    "freq_ghz": 2.4
+  },
+  "accuracy": {
+    "within_room_1": 1.0,
+    "within_room_2": 1.0,
+    "cross_room_raw": 0.1,
+    "cross_room_meridian_labelled": 0.1,
+    "cross_room_physics_informed": 0.1,
+    "chance": 0.1
+  }
+}
\ No newline at end of file
diff --git a/examples/research-sota/r3_1_physics_informed_env.py b/examples/research-sota/r3_1_physics_informed_env.py
new file mode 100644
index 00000000..aff12704
--- /dev/null
+++ b/examples/research-sota/r3_1_physics_informed_env.py
@@ -0,0 +1,222 @@
+#!/usr/bin/env python3
+"""R3.1 — Physics-informed env_sig prediction for zero-shot cross-room re-ID.
+
+See docs/research/sota-2026-05-22/R3_1-physics-informed-env-prediction.md.
+
+R3 showed MERIDIAN env-centroid subtraction recovers cross-room re-ID
+accuracy, but requires labelled examples IN THE NEW ROOM to estimate
+the per-room centroid. The "next research lever" identified in R3:
+
+  Use R6.1 forward operator + a coarse room map to PREDICT the env_sig
+  without labelled examples.
+
+This tick implements that. Two rooms (5x5 and 4x6) with different wall
+reflector configurations. For each room, we:
+
+  1. Compute predicted env_sig from R6.1 forward model summed over the
+     room's wall scatterers (no person).
+  2. For each subject's CSI in that room, subtract the predicted env_sig
+     before doing K-NN matching.
+  3. Compare to MERIDIAN-with-labels (oracle baseline) and raw cross-room.
+
+The goal: how close can physics-informed env prediction get to
+MERIDIAN, with ZERO labelled examples in the new room?
+
+Pure NumPy.
+"""
+
+from __future__ import annotations
+
+import argparse
+import json
+from pathlib import Path
+import numpy as np
+
+C = 2.998e8
+
+
+def wavelength_m(freq_ghz: float) -> float:
+    return C / (freq_ghz * 1e9)
+
+
+def csi_contribution(scatterer_pos, reflectivity, tx_pos, rx_pos, sub_freqs_hz):
+    d_tx = np.linalg.norm(scatterer_pos - tx_pos)
+    d_rx = np.linalg.norm(scatterer_pos - rx_pos)
+    d_direct = np.linalg.norm(tx_pos - rx_pos)
+    delta_l = d_tx + d_rx - d_direct
+    amp = reflectivity / max(d_tx * d_rx, 1e-3)
+    phase = 2 * np.pi * sub_freqs_hz * delta_l / C
+    return amp * np.exp(1j * phase)
+
+
+def simulate(scatterers, tx, rx, freq_ghz, n_sub=52, sub_spacing_khz=312.5):
+    sub_offsets = (np.arange(n_sub) - n_sub // 2) * sub_spacing_khz * 1e3
+    sub_freqs = freq_ghz * 1e9 + sub_offsets
+    total = np.zeros(n_sub, dtype=complex)
+    for s in scatterers:
+        total += csi_contribution(np.asarray(s["pos"]), s["refl"],
+                                 np.asarray(tx), np.asarray(rx), sub_freqs)
+    return total
+
+
+def human_body(cx, cy, person_scale=1.0):
+    """Person scale slightly varies between subjects (body size).
+    Returns list of 6 body-part scatterers."""
+    return [
+        {"pos": [cx,        cy       ], "refl": 0.10 * person_scale, "name": "head"},
+        {"pos": [cx,        cy       ], "refl": 0.50 * person_scale, "name": "chest"},
+        {"pos": [cx - 0.20*person_scale, cy], "refl": 0.10 * person_scale, "name": "left_arm"},
+        {"pos": [cx + 0.20*person_scale, cy], "refl": 0.10 * person_scale, "name": "right_arm"},
+        {"pos": [cx - 0.10*person_scale, cy - 0.40*person_scale], "refl": 0.10 * person_scale, "name": "l_leg"},
+        {"pos": [cx + 0.10*person_scale, cy - 0.40*person_scale], "refl": 0.10 * person_scale, "name": "r_leg"},
+    ]
+
+
+def room_walls_5x5():
+    """Bedroom: square 5x5m with 4 wall scatterers."""
+    return [
+        {"pos": [0.5, 4.5], "refl": 0.30},
+        {"pos": [4.5, 4.5], "refl": 0.25},
+        {"pos": [0.5, 0.5], "refl": 0.20},
+        {"pos": [4.5, 0.5], "refl": 0.15},
+    ]
+
+
+def room_walls_4x6():
+    """Living room: 4x6m with 4 wall scatterers in different positions/refl."""
+    return [
+        {"pos": [0.3, 5.7], "refl": 0.28},
+        {"pos": [3.7, 5.7], "refl": 0.18},
+        {"pos": [0.3, 0.3], "refl": 0.32},
+        {"pos": [3.7, 0.3], "refl": 0.22},
+    ]
+
+
+def cosine_dist(a, b):
+    norm_a = np.linalg.norm(a)
+    norm_b = np.linalg.norm(b)
+    if norm_a < 1e-9 or norm_b < 1e-9: return 1.0
+    return 1.0 - float(np.real(np.vdot(a, b) / (norm_a * norm_b)))
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--out", default="examples/research-sota/r3_1_physics_env_results.json")
+    args = parser.parse_args()
+
+    freq = 2.4
+    # Subjects: 10 individuals with slightly varying body sizes
+    n_subj = 10
+    rng = np.random.default_rng(42)
+    body_scales = 0.85 + 0.30 * rng.random(n_subj)  # 0.85 to 1.15
+
+    # Room 1: 5x5, link goes diagonally (per R6.2 best placement)
+    room1_walls = room_walls_5x5()
+    tx1, rx1 = np.array([1.25, 0.0]), np.array([4.75, 5.0])
+    room1_subject_positions = [(2.5, 2.75), (2.5, 2.5), (2.0, 3.0)]  # 3 positions
+    # Room 2: 4x6, different geometry
+    room2_walls = room_walls_4x6()
+    tx2, rx2 = np.array([1.0, 0.0]), np.array([3.0, 6.0])
+    room2_subject_positions = [(2.0, 3.0), (1.5, 3.5), (2.5, 2.5)]
+
+    # === Step 1: PREDICTED env_sig from physics (no labels needed) ===
+    # Just simulate the room with NO subject -- this is what the empty
+    # room "looks like" to the antennas.
+    env_sig_room1_predicted = simulate(room1_walls, tx1, rx1, freq)
+    env_sig_room2_predicted = simulate(room2_walls, tx2, rx2, freq)
+
+    # === Step 2: Generate CSI per subject in each room ===
+    csi_room1, csi_room2 = [], []
+    for i in range(n_subj):
+        scale = body_scales[i]
+        for pos in room1_subject_positions:
+            body = human_body(*pos, person_scale=scale)
+            scene = body + room1_walls
+            csi_room1.append(simulate(scene, tx1, rx1, freq))
+        for pos in room2_subject_positions:
+            body = human_body(*pos, person_scale=scale)
+            scene = body + room2_walls
+            csi_room2.append(simulate(scene, tx2, rx2, freq))
+    csi_room1 = np.array(csi_room1)
+    csi_room2 = np.array(csi_room2)
+    labels = np.repeat(np.arange(n_subj), len(room1_subject_positions))
+
+    # === Step 3: Compute the LABELED MERIDIAN centroid (oracle baseline) ===
+    centroid_room1_meridian = csi_room1.mean(axis=0)
+    centroid_room2_meridian = csi_room2.mean(axis=0)
+
+    # === Step 4: Cross-room re-ID with three approaches ===
+
+    def knn_accuracy(query, gallery, q_labels, g_labels, k=1):
+        correct = 0
+        for i in range(len(query)):
+            dists = [cosine_dist(query[i], g) for g in gallery]
+            top_k = np.argsort(dists)[:k]
+            top_k_labels = [g_labels[j] for j in top_k]
+            vals, counts = np.unique(top_k_labels, return_counts=True)
+            pred = vals[np.argmax(counts)]
+            if pred == q_labels[i]:
+                correct += 1
+        return correct / len(query)
+
+    # Gallery = room 1 (train), Query = room 2 (test)
+    # (a) Raw cross-room
+    acc_raw = knn_accuracy(csi_room2, csi_room1, labels, labels)
+
+    # (b) MERIDIAN with labelled centroid (oracle)
+    csi_room1_cleaned = csi_room1 - centroid_room1_meridian
+    csi_room2_cleaned = csi_room2 - centroid_room2_meridian
+    acc_meridian = knn_accuracy(csi_room2_cleaned, csi_room1_cleaned, labels, labels)
+
+    # (c) Physics-informed env prediction (ZERO labels in either room)
+    csi_room1_phys = csi_room1 - env_sig_room1_predicted
+    csi_room2_phys = csi_room2 - env_sig_room2_predicted
+    acc_physics = knn_accuracy(csi_room2_phys, csi_room1_phys, labels, labels)
+
+    # === Within-room baselines ===
+    acc_within_room1 = knn_accuracy(csi_room1, csi_room1, labels, labels)
+    acc_within_room2 = knn_accuracy(csi_room2, csi_room2, labels, labels)
+
+    out = {
+        "config": {
+            "n_subjects": n_subj,
+            "n_positions_per_room": len(room1_subject_positions),
+            "rooms": ["5x5 m", "4x6 m"],
+            "freq_ghz": freq,
+        },
+        "accuracy": {
+            "within_room_1": acc_within_room1,
+            "within_room_2": acc_within_room2,
+            "cross_room_raw": acc_raw,
+            "cross_room_meridian_labelled": acc_meridian,
+            "cross_room_physics_informed": acc_physics,
+            "chance": 1.0 / n_subj,
+        },
+    }
+    Path(args.out).parent.mkdir(parents=True, exist_ok=True)
+    Path(args.out).write_text(json.dumps(out, indent=2))
+
+    print("=== R3.1 physics-informed env_sig prediction ===")
+    print(f"  {n_subj} subjects, {len(room1_subject_positions)} positions per room")
+    print(f"  Room 1 (5x5 m, diagonal link) vs Room 2 (4x6 m, different geometry)")
+    print()
+    print(f"=== 1-shot K-NN re-ID accuracy ===")
+    print(f"  Within-room 1 baseline:                {acc_within_room1*100:6.1f}%")
+    print(f"  Within-room 2 baseline:                {acc_within_room2*100:6.1f}%")
+    print(f"  Cross-room RAW (no env subtraction):   {acc_raw*100:6.1f}%")
+    print(f"  Cross-room MERIDIAN (labelled oracle): {acc_meridian*100:6.1f}%")
+    print(f"  Cross-room PHYSICS-INFORMED:           {acc_physics*100:6.1f}%  (this tick)")
+    print(f"  Chance:                                {100/n_subj:6.1f}%")
+    print()
+    if acc_physics >= acc_meridian * 0.9:
+        print(f"VERDICT: physics-informed matches MERIDIAN within 10% with ZERO labels in either room.")
+    elif acc_physics > acc_raw * 1.5:
+        print(f"VERDICT: physics-informed lifts cross-room accuracy {acc_physics/acc_raw:.1f}x vs raw.")
+    else:
+        print(f"VERDICT: physics-informed only modestly improves over raw; needs refinement.")
+    print()
+    print(f"Wrote {args.out}")
+
+
+if __name__ == "__main__":
+    main()