Add overlay extraction for multi-expansion asset deduplication

Extracts each expansion's assets as a CRC-compared overlay against a
base manifest, storing only files that differ. Auto-detects overlay mode
when a base manifest already exists. Adds --as-overlay, --full-base
flags and manifest merge for partial extractions.

tools/asset_extract/extractor.hpp

@@ -26,6 +26,7 @@ public:
         bool onlyUsedDbcs = false; // Extract only the DBC files wowee uses (implies DBFilesClient/*.dbc filter)
         std::string dbcCsvOutputDir; // When set, write CSVs into this directory instead of outputDir/expansions/<exp>/db
         std::string referenceManifest; // If set, only extract files NOT in this manifest (delta extraction)
+        std::string asOverlay; // If set, extract as overlay for this expansion ID (only files differing from base)
     };
 
     struct Stats {