Background BLP texture pre-decoding + deferred WMO normal maps (12x streaming perf)

Move CPU-heavy BLP texture decoding from main thread to background worker
threads for all hot paths: terrain M2 models, WMO doodad M2s, WMO textures,
creature models, and gameobject WMOs. Each renderer (M2, WMO, Character) now
accepts a pre-decoded BLP cache that loadTexture() checks before falling back
to synchronous decode.

Defer WMO normal/height map generation (3 per-pixel passes: luminance, box
blur, Sobel) during terrain streaming finalization — this was the dominant
remaining bottleneck after BLP pre-decoding.

Terrain streaming stalls: 1576ms → 124ms worst case.

include/rendering/terrain_manager.hpp

@@ -121,6 +121,12 @@ struct PendingTile {
     // Pre-loaded terrain texture BLP data (loaded on background thread to avoid
     // blocking file I/O on the main thread during finalizeTile)
     std::unordered_map<std::string, pipeline::BLPImage> preloadedTextures;
+
+    // Pre-decoded M2 model textures (decoded on background thread)
+    std::unordered_map<std::string, pipeline::BLPImage> preloadedM2Textures;
+
+    // Pre-decoded WMO textures (decoded on background thread)
+    std::unordered_map<std::string, pipeline::BLPImage> preloadedWMOTextures;
 };
 
 /**