Add purgeExtractedAssets() to clear all extracted asset data from disk

include/pipeline/asset_manager.hpp

@@ -117,6 +117,15 @@ public:
      */
     void clearDBCCache();
 
+    /**
+     * Delete all extracted asset files from the data directory on disk.
+     * Removes extracted subdirectories (db, character, creature, terrain, etc.),
+     * manifest.json, override dir, and expansion-specific extracted assets.
+     * After calling this, initialize() will fail until assets are re-extracted.
+     * @return Number of entries removed
+     */
+    size_t purgeExtractedAssets();
+
 private:
     bool initialized = false;
     std::string dataPath;