Add 1GB RAM cache for decompressed MPQ files

Eliminates repeated MPQ decompression overhead by caching decompressed
files in RAM with LRU eviction. Major performance improvement for file access.

Problem:
- Every readFile() call decompresses from MPQ (expensive!)
- M2 models, textures, WMO files decompressed repeatedly
- No caching of decompressed data
- MPQ decompression is CPU-intensive (zlib/bzip2)

Solution:
- Added 1GB LRU file cache to AssetManager
- Cache hit: instant return of decompressed data
- Cache miss: decompress once, cache for future access
- LRU eviction when cache full (removes least recently used)
- Don't cache files >100MB (avoid giant WMO chunks)
- Thread-safe with existing readMutex

Implementation:
- CachedFile struct: data + lastAccessTime
- fileCacheAccessCounter for LRU tracking
- Hit/miss statistics for monitoring
- Budget: 1GB (modern RAM easily handles this)

Performance impact:
- First load: same speed (decompress + cache)
- Subsequent loads: instant (no decompression)
- Expected 70-90% hit rate during normal play
- Huge benefit for frequently accessed models

Cache stats logged on shutdown to monitor effectiveness.

include/pipeline/asset_manager.hpp

@@ -84,6 +84,13 @@ public:
      */
     size_t getLoadedDBCCount() const { return dbcCache.size(); }
 
+    /**
+     * Get file cache stats
+     */
+    size_t getFileCacheSize() const { return fileCacheTotalBytes; }
+    size_t getFileCacheHits() const { return fileCacheHits; }
+    size_t getFileCacheMisses() const { return fileCacheMisses; }
+
     /**
      * Clear all cached resources
      */
@@ -97,6 +104,18 @@ private:
     mutable std::mutex readMutex;
     std::map<std::string, std::shared_ptr<DBCFile>> dbcCache;
 
+    // Decompressed file cache (LRU, 1GB budget for modern RAM)
+    struct CachedFile {
+        std::vector<uint8_t> data;
+        uint64_t lastAccessTime;
+    };
+    mutable std::map<std::string, CachedFile> fileCache;
+    mutable size_t fileCacheTotalBytes = 0;
+    mutable uint64_t fileCacheAccessCounter = 0;
+    mutable size_t fileCacheHits = 0;
+    mutable size_t fileCacheMisses = 0;
+    static constexpr size_t FILE_CACHE_BUDGET = 1024 * 1024 * 1024;  // 1GB
+
     /**
      * Normalize path for case-insensitive lookup
      */