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.

include/game/expansion_profile.hpp

@@ -30,7 +30,6 @@ struct ExpansionProfile {
     std::string locale = "enUS";
     uint32_t timezone = 0;
     std::string dataPath;      // Absolute path to expansion data dir
-    std::string assetManifest;  // Path to expansion-specific asset manifest (absolute, resolved from dataPath)
     uint32_t maxLevel = 60;
     std::vector<uint32_t> races;
     std::vector<uint32_t> classes;