fix(physics): activate collision generations atomically

memory/project_collision_port.md

@@ -10,6 +10,14 @@ as "delete everything and start over." A partial retail transition port exists:
 - `TransitionTypes` carries the active `SpherePath`, `CollisionInfo`,
   transition, step, contact, and partial slide logic.
 - `PhysicsDataCache` loads GfxObj, Setup, and CellStruct physics data.
+- Landblock streaming never edits the live collision world incrementally.
+  `PreparedLandblockCollisionGeneration` owns an off-side cache, CellGraph,
+  engine snapshot, buildings, statics, and versioned dynamic-owner refloods;
+  `RuntimePhysicsState.CommitCollisionGeneration` validates freshness and
+  activates the complete generation on the single update thread. A dirty live
+  owner returns to the App cursor for refresh. The old generation remains
+  queryable until that commit. See
+  `docs/research/2026-07-31-atomic-collision-generation.md`.
 - `ShadowObjectRegistry` gives the resolver a broadphase over nearby objects.
 - `TerrainSurface` uses triangle-aware terrain contact.