The 30↔200 FPS swing is _datLock contention: the streaming worker holds
the global dat lock for the full per-landblock build (lockwait measured
24ms median / 88ms p95), stalling the update thread's ApplyLoadedTerrain.
Fix (A1): the worker pre-reads the apply's six Get<T> sites into a physics
dat bundle so ApplyLoadedTerrainLocked makes zero DatCollection calls and
its lock(_datLock) is removed. Approved design; implementation next.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
