Fix M2 texture loading, /unstuckgy, and WMO floor detection

- Add mutex to AssetManager::loadTexture/loadDBC/fileExists to prevent
  StormLib thread-safety races that silently fail texture reads; stop
  caching texture load failures so transient errors are retried.
- Replace /unstuckgy DBC lookup (which used wrong coordinate transform)
  with hardcoded safe locations per map.
- Widen WMO floor raycast from single grid cell to ±1 unit range query
  to catch bridge/walkway triangles at cell boundaries.
- Tighten swept collision hit threshold (0.5 → 0.15) and grid query
  margin (2.5 → 1.5) to prevent false-positive wall pushes.
- Tighten post-wall-push Z snap lower bound (-1.0 → -0.3) to prevent
  gradual floor sinking.

src/rendering/camera_controller.cpp

@@ -481,7 +481,7 @@ void CameraController::update(float deltaTime) {
                                 candidate.y = adjusted.y;
                                 // Snap Z to floor at adjusted position to prevent fall-through
                                 auto adjFloor = wmoRenderer->getFloorHeight(adjusted.x, adjusted.y, feetZ + 2.5f);
-                                if (adjFloor && *adjFloor >= feetZ - 1.0f && *adjFloor <= feetZ + 1.6f) {
+                                if (adjFloor && *adjFloor >= feetZ - 0.3f && *adjFloor <= feetZ + 1.6f) {
                                     candidate.z = *adjFloor;
                                 }
                             } else if (floorH && *floorH > candidate.z) {