Add per-expansion asset overlay system and fix CharSections DBC layout

Expansion overlays allow each expansion to supplement the base asset data
via an assetManifest field in expansion.json, loaded at priority 50 (below
HD packs). The asset extractor gains --reference-manifest for delta-only
extraction. Also fixes CharSections field indices (VariationIndex=4,
ColorIndex=5, Texture1=6) across all DBC layout references.

tools/asset_extract/extractor.hpp

@@ -25,6 +25,7 @@ public:
         bool skipDbcExtraction = false; // Extract visual assets only (recommended when CSV DBCs are in repo)
         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)
     };
 
     struct Stats {