Fix city stuttering with incremental tile finalization and GPU optimizations

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- Remove dead PendingM2Upload queue code superseded by incremental system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.

include/rendering/wmo_renderer.hpp

@@ -657,9 +657,9 @@ private:
     bool wireframeMode = false;
     bool frustumCulling = true;
     bool portalCulling = false;  // Disabled by default - needs debugging
-    bool distanceCulling = false;  // Disabled - causes ground to disappear
-    float maxGroupDistance = 500.0f;
-    float maxGroupDistanceSq = 250000.0f;  // maxGroupDistance^2
+    bool distanceCulling = true;   // Enabled with active-group exemption to prevent floor disappearing
+    float maxGroupDistance = 800.0f;
+    float maxGroupDistanceSq = 640000.0f;  // maxGroupDistance^2
     uint32_t lastDrawCalls = 0;
     mutable uint32_t lastPortalCulledGroups = 0;
     mutable uint32_t lastDistanceCulledGroups = 0;