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.

include/core/application.hpp

@@ -216,6 +216,8 @@ private:
     bool wasAutoAttacking_ = false;
     void processPendingMount();
     bool creatureLookupsBuilt_ = false;
+    bool mapNameCacheLoaded_ = false;
+    std::unordered_map<uint32_t, std::string> mapNameById_;
 
     // Deferred creature spawn queue (throttles spawning to avoid hangs)
     struct PendingCreatureSpawn {