feat(physics): S2 chunk 4 - movement publishes from the transition's cells; children inherit at registration

Movement: CommitSetPosition's RefreshPositionRows/ReplacePositionRows and
the staged apply publish the retail render product from the exact cell
list collision just used, the transition's cell_array retail feeds
add_shadows_to_cells in CPhysicsObj::SetPositionInternal @0x00515330
(pseudo-C 283526-283539); the separate move-path bbox recompute is deleted.
calc_cross_cells @0x00515230 stays the distinct full-recompute path
(PhysicsShadowCommitAction.Recalculate).

Children (Contract B recursion): ShadowObjectRegistry.AttachChild/DetachChild
give an attached object the root's current cells as part entries only,
republished whenever the root's array changes, detached at withdrawal and
cascaded from the root's Deregister; nested attachment resolves to the root
with a bounded, cycle-safe chain. EquippedChildRenderController attaches at
realization (FromSetupRenderParts over the child's Setup) and detaches at
its single removal funnel. WalkProductionWorldData's dynamic sweep reads
TryGetRetailCellArray directly; the 64-hop parent-chain walk and its
FindParentLocalId plumbing are deleted. CollisionWorldState.Clear now
also clears the retail products.

Gates (implementer's isolated worktree at identical content): Release
build 0/0; Core 4,970/4,970; App hermetic 6,761/6,761; targeted
walk/child/live-entity/placement/comparator 166/166; Runtime 1,884/1,884.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/AcDream.App.Tests/Physics/LiveEntityNetworkOnPositionCollapseMatrixTests.cs

@@ -1388,7 +1388,9 @@ public sealed class LiveEntityNetworkOnPositionCollapseMatrixTests
                     static (_, _, _) =>
                         new ExactProjectionWithdrawalOutcome(
                             ExactProjectionWithdrawalDisposition.Superseded,
-                            null)),
+                            null),
+                    Shadows,
+                    new PhysicsDataCache()),
                 new ProjectileController(Runtime),
                 animatedEntities,
                 new RemoteMovementObservationTracker(),