Unify asset system: one asset set, always high-res

Remove HDPackManager, expansion overlay manifests, and BLP size-comparison
logic. Assets now resolve through a single manifest with a simple override
directory (Data/override/) for future HD upgrades.

tools/asset_extract/extractor.hpp

@@ -26,7 +26,6 @@ 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 {