Automate WMO floor cache pre-computation

Add precomputeFloorCache() method that samples all WMO bounds at load time
and populates the persistent floor height grid. Called after terrain
streaming completes (initial spawn and teleport) when cache is empty.

include/rendering/wmo_renderer.hpp

@@ -227,6 +227,9 @@ public:
     bool loadFloorCache(const std::string& filepath);
     size_t getFloorCacheSize() const { return precomputedFloorGrid.size(); }
 
+    // Pre-compute floor cache for all loaded WMO instances
+    void precomputeFloorCache();
+
 private:
     /**
      * WMO group GPU resources