fix(physics): activate collision generations atomically

docs/architecture/acdream-architecture.md

@@ -485,8 +485,21 @@ What exists and is active:
   collision assets loaded from the validated prepared package. Production
   retains no parsed DAT collision graph; graph construction is restricted to
   bake/equivalence tools and explicit test oracles.
-- `ShadowObjectRegistry` gives movement a broadphase over nearby objects and
-  buildings.
+- Landblock collision activation is generation-owned by
+  `RuntimePhysicsState`. Graphical and no-window hosts populate a private
+  `PreparedLandblockCollisionGeneration` over bounded cursors; its cache,
+  `CellGraph`, engine landblock, buildings, static shadows, and dynamic-owner
+  refloods are never visible through the borrowed live engine. Runtime validates
+  the exact admission plus every affected dynamic shadow-owner version, seals
+  the complete replacement, and commits it synchronously on the one update
+  thread before emitting `CollisionGenerationCommitted`. A moving, spawned, or
+  deleted owner makes the generation not-ready and returns only that dirty owner
+  to the cursor-budgeted refresh phase. Thus readers see the complete old
+  generation or complete new generation, never a mixed cell/cache/shadow world.
+- `ShadowObjectRegistry` gives movement a per-cell broadphase over nearby
+  objects and buildings. Streaming reflood is structurally part of the Runtime
+  collision-generation commit; there is no independent post-publication
+  reflood suffix.
 - `TerrainSurface` uses triangle-aware terrain contact; older "bilinear terrain
   Z" descriptions are historical B.3 language, not current architecture.