Replace monolithic finalizeTile() with a phased state machine that spreads
GPU upload work across multiple frames (TERRAIN→M2→WMO→WATER→AMBIENT→DONE).
Each advanceFinalization() call does one bounded unit of work within the
per-frame time budget, eliminating 50-300ms frame hitches when entering cities.
Additional performance improvements:
- Pre-allocate bone SSBOs at M2 instance creation instead of lazily during
first render frame, preventing hitches when many skinned characters appear
- Enable WMO distance culling (800 units) with active-group exemption so
the player's current floor/neighbors are never culled
- Add 4-tier adaptive M2 render distance (250/400/600/1000 based on count)
- Remove dead PendingM2Upload queue code superseded by incremental system
Fix tile re-enqueueing bug: keep tiles in pendingTiles until committed to
loadedTiles (not when moved to finalizingTiles_) so streamTiles() doesn't
re-enqueue tiles mid-finalization. Also handle unloadTile() for tiles in
the finalizingTiles_ deque to prevent orphaned water/M2/WMO resources.