Adds AabbMin/AabbMax (per-entity world-space bounding box) and AabbDirty
flag to WorldEntity. RefreshAabb() recomputes the box from Position ±5 m
(DefaultAabbRadius). SetPosition() writes Position and marks the cache
dirty so the dispatcher calls RefreshAabb on first read rather than
carrying stale bounds.
AabbDirty defaults to true on construction — freshly-built entities have
zero AabbMin/AabbMax until RefreshAabb is called. Two new conformance tests
verify the ±5 m geometry and the dirty/clean state machine.
Per Phase A.5 spec §4.6 Change #2.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
