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/rendering/m2_renderer.hpp

@@ -475,9 +475,7 @@ private:
     static constexpr float SPATIAL_CELL_SIZE = 64.0f;
     std::unordered_map<GridCell, std::vector<uint32_t>, GridCellHash> spatialGrid;
     std::unordered_map<uint32_t, size_t> instanceIndexById;
-    mutable std::vector<size_t> candidateScratch;
-    mutable std::unordered_set<uint32_t> candidateIdScratch;
-    mutable std::vector<uint32_t> collisionTriScratch_;
+    // Collision scratch buffers are thread_local (see m2_renderer.cpp) for thread-safety.
 
     // Collision query profiling (per frame).
     mutable double queryTimeMs = 0.0;

include/rendering/renderer.hpp

@@ -246,7 +246,7 @@ private:
     glm::vec3 shadowCenter = glm::vec3(0.0f);
     bool shadowCenterInitialized = false;
     bool shadowsEnabled = true;
-    float shadowDistance_ = 72.0f;  // Shadow frustum half-extent (default: 72 units)
+    float shadowDistance_ = 300.0f;  // Shadow frustum half-extent (default: 300 units)
     uint32_t shadowFrameCounter_ = 0;
 
 
@@ -257,7 +257,7 @@ public:
 
     void setShadowsEnabled(bool enabled) { shadowsEnabled = enabled; }
     bool areShadowsEnabled() const { return shadowsEnabled; }
-    void setShadowDistance(float dist) { shadowDistance_ = glm::clamp(dist, 40.0f, 200.0f); }
+    void setShadowDistance(float dist) { shadowDistance_ = glm::clamp(dist, 40.0f, 500.0f); }
     float getShadowDistance() const { return shadowDistance_; }
     void setMsaaSamples(VkSampleCountFlagBits samples);
 

include/rendering/wmo_renderer.hpp

@@ -711,9 +711,7 @@ private:
     static constexpr float SPATIAL_CELL_SIZE = 64.0f;
     std::unordered_map<GridCell, std::vector<uint32_t>, GridCellHash> spatialGrid;
     std::unordered_map<uint32_t, size_t> instanceIndexById;
-    mutable std::vector<size_t> candidateScratch;
-    mutable std::vector<uint32_t> triScratch_;  // Scratch for collision grid queries
-    mutable std::unordered_set<uint32_t> candidateIdScratch;
+    // Collision scratch buffers are thread_local (see wmo_renderer.cpp) for thread-safety.
 
     // Parallel visibility culling
     uint32_t numCullThreads_ = 1;