Iter 24 — ultra-opt for public-API stability. Iter 23 added a new JSON
field that UI clients (viz.html, future Tauri desktop, automation) now
depend on; this iter locks its exact shape so any future rename /
removal fails a named test instead of silently breaking consumers.
New module `node_sync_snapshot_serialization_tests` (3 tests, all green):
* sync_present_serializes_all_seven_fields
Builds NodeInfo with Some(sample_sync), serializes to serde_json::Value,
asserts all 7 documented field names exist (offset_us, is_leader,
is_valid, smoothed, sequence, csi_fps_ema, csi_fps_samples) and
spot-checks numeric values.
* sync_absent_omits_the_key_entirely
Builds NodeInfo with sync = None, asserts the `sync` JSON key is
DROPPED entirely (not emitted as `"sync": null`). This is the
backwards-compat contract that lets pre-iter-23 UI clients ignore
mesh-aware nodes silently.
* sync_round_trips_through_serde
to_string / from_str round-trip on a populated NodeInfo recovers
every field of the sync sub-object byte-for-byte (modulo float tol).
Test infrastructure: pure pure serde_json — no network, no fixtures,
no I/O. Adds 92 lines, 0 runtime allocs in the steady path.
Branch-coord clean (no Cargo.toml or cli.rs touched).
Co-Authored-By: claude-flow <ruv@ruv.net>
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