Parallel animation updates, thread-safe collision, M2 pop-in fix, shadow stabilization

- Overlap M2 and character animation updates via std::async (~2-5ms saved)
- Thread-local collision scratch buffers for concurrent floor queries
- Parallel terrain/WMO/M2 floor queries in camera controller
- Seed new M2 instance bones from existing siblings to eliminate pop-in flash
- Fix shadow flicker: snap center along stable light axes instead of in view space
- Increase shadow distance default to 300 units (slider max 500)

include/ui/game_screen.hpp

@@ -87,7 +87,7 @@ private:
     bool pendingVsync = false;
     int pendingResIndex = 0;
     bool pendingShadows = true;
-    float pendingShadowDistance = 72.0f;
+    float pendingShadowDistance = 300.0f;
     bool pendingWaterRefraction = false;
     int pendingMasterVolume = 100;
     int pendingMusicVolume = 30;