phase: 06_realtime_websocket_protocol

All green. Final verification complete — writing the report.

**Final pass report — phase 06 `realtime_websocket_protocol`**

- Verified: all 6 task files present in `complete/`; this was the final verification pass (no implementation work outstanding)
- `uv run ruff check .` → all checks passed; `uv run pyright` → 0 errors
- `uv run undercooked-levels validate` → content OK; `uv run pytest` (full suite) → **1751 passed, 0 failed, coverage 97.30% (>90% floor)**; slow sim stage (`-m slow tests/sim --no-cov`) → 2 passed; repo hygiene clean
- App launches: `/healthz` 200, `/` 200, `/api/levels`, `/api/config`, `POST /api/rooms` (201), `/api/stats` all verified live on an ephemeral port (server cleaned up)
- Criteria: scripted client room→hello→ready→start→countdown→≥20 snaps→round — PASS (`test_ws_round_two_clients.py`); identical `round` payloads both clients — PASS (`round_a == round_b`); version mismatch → `err protocol_mismatch` + close `4400` — PASS; 64 KB refused / 4 KB boundary, 500-burst → `rate_limited` → `4404` with bounded queue, 2000 malformed + 200 handshakes within memory bound — PASS; snap ≤ 1.5 KB (4p/4o/3h) and 64×64 ≤ 4 KB, downlink ≤ 60 KB/s over 100 ticks — PASS; `docs/PROTOCOL.md` model-sync test-enforced in both directions + full-round sequence diagram present — PASS
- Docs: README.md had 3 stale spots — status section (claimed 01–05 complete, 06 pending → now 01–06 complete, 07 pending), "until phases 06/07 wire the transport" wording, and the `PROTOCOL.md` layout note ("phase 06 completes it"). All fixed. AGENTS.md verified consistent — no changes needed
- No defects found in completed phases; no live infrastructure touched

Next pending phase: `07_canvas_client_and_hud`.

tests/integration/test_room_isolation.py

@@ -287,11 +287,16 @@ def test_one_room_fails_without_affecting_the_other(
         bring_both_to_playing(room_a, room_b, clock)
 
         inject_the_fault(room_a)
-        # The crash tick — isolated: it must not propagate. The wall
-        # time is not advanced: A takes this tick at the last wall time
-        # (dt 0), so B's cadence below stays exactly one 0.05 s frame
-        # per substep.
+        # The crash tick — isolated: it must not propagate. The task-03
+        # substep guard makes a dt-0 tick skip `step` entirely (a
+        # zero-substep tick would drop queued inputs without applying
+        # them), so the fault needs a substep-advancing tick: advance
+        # the wall clock by one period. B's `_last_at` is re-anchored
+        # to the same wall time so B's cadence below stays exactly one
+        # 0.05 s frame per substep.
+        clock.advance(PERIOD)
         room_a.tick_once()
+        room_b._last_at = clock.now()
 
         check_room_a_is_aborted(room_a, caplog, ada.token, bob.token)
         check_room_b_keeps_ticking(room_b, clock)