Preload terrain textures on background thread and fix ramp Z-snapping

Load BLP texture data during prepareTile() and upload to GL cache in
finalizeTile(), eliminating file I/O stalls on the main thread. Reduce
ready tiles per frame to 1. Fix camera sweep to snap Z to ramp surfaces.
Change hearthstone action bar slot from spell to item.

include/rendering/terrain_manager.hpp

@@ -4,6 +4,7 @@
 #include "pipeline/terrain_mesh.hpp"
 #include "pipeline/m2_loader.hpp"
 #include "pipeline/wmo_loader.hpp"
+#include "pipeline/blp_loader.hpp"
 #include <string>
 #include <unordered_map>
 #include <unordered_set>
@@ -103,6 +104,10 @@ struct PendingTile {
         glm::mat4 modelMatrix;     // Pre-computed world transform
     };
     std::vector<WMODoodadReady> wmoDoodads;
+
+    // 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;
 };
 
 /**

include/rendering/terrain_renderer.hpp

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "pipeline/terrain_mesh.hpp"
+#include "pipeline/blp_loader.hpp"
 #include "rendering/shader.hpp"
 #include "rendering/texture.hpp"
 #include "rendering/camera.hpp"
@@ -87,6 +88,12 @@ public:
      */
     void removeTile(int tileX, int tileY);
 
+    /**
+     * Upload pre-loaded BLP textures to the GL texture cache.
+     * Called before loadTerrain() so texture loading avoids file I/O.
+     */
+    void uploadPreloadedTextures(const std::unordered_map<std::string, pipeline::BLPImage>& textures);
+
     /**
      * Render loaded terrain
      * @param camera Camera for view/projection matrices