research: region completion gate (L123, no production change)

The 18 ms gate is one game-thread tick, not compile cost (0.25 ms). MPSC handoff keeps ownership but still
polls per tick; velocity look-ahead didn't help (near -0.8 pt, p99 11.77 > 11.00). Both kept as a patch.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016dhxM43dnWvUu2B5Ph1iG1

Standalone/Research/WorldAssemblerM3/Linux123/REPORT.md

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+# L123: region completion gate
+
+Five interleaved 30 m/s UE pairs show the gate is a game-thread poll delay. Core compile wall time is p50 0.251 ms (worker queue plus task p50 0.475 ms); request-to-observe is p50 18.89 ms, p95 21.68 ms. In 95% of traced completions, the next scheduler pass drains the MPSC result and resumes admission in that same pass; a small tail waits 2–4 scheduler frames. This is one tick interval, not separate compile, observe, and admit ticks.
+
+The worker can safely hand off an immutable result through MPSC with its runtime state and `GenerationEpoch`. The game thread validates both before updating state and admitting tiles. Direct worker-side admission would touch game-thread state unsafely. This queue handoff preserves ownership but still polls once per tick; a game-thread wakeup/continuation is the safe way to try removing that delay. A wakeup was not timed in this round.
+
+One-step velocity lookahead did not remove the wait (gate p50 19.0 ms). Paired near readiness fell 0.79 percentage points; paired far holes rose 0.14 points. Its worst-run p99 was 11.77 ms (limit 11.00) and worst tick 15.11 ms (limit 14.74). After a 90° turn, 252 of 960 retained ahead keys became stale. At 1.4 m/s it compiled 128 ahead tiles per run; paired near arrival was 2.6 ms faster, while far-clear arrival was 14 ms slower. Keep lookahead off.
+
+The queue-only runs also missed the p99 limit (worst 11.46 ms), though their worst tick was 14.19 ms. Neither measured option meets the full bar. Headless 30 m/s completed with zero generation failures (82.71% near readiness, 14.21% far holes); this path has no UE region-completion gate. Geometry checks passed 24/24 for Clang/GCC W1/W8/W12, and the fast tier passed.