Add multi-expansion support with data-driven protocol layer

Replace hardcoded WotLK protocol constants with a data-driven architecture
supporting Classic 1.12.1, TBC 2.4.3, and WotLK 3.3.5a. Each expansion
has JSON profiles for opcodes, update fields, and DBC layouts, plus C++
polymorphic packet parsers for binary format differences (movement flags,
speed fields, transport data, spline format, char enum layout).

Key components:
- ExpansionRegistry: scans Data/expansions/*/expansion.json at startup
- OpcodeTable: logical enum <-> wire values loaded from JSON
- UpdateFieldTable: field indices loaded from JSON per expansion
- DBCLayout: schema-driven DBC field lookups replacing magic numbers
- PacketParsers: WotLK/TBC/Classic parsers with correct flag positions
- Multi-manifest AssetManager: layered manifests with priority ordering
- HDPackManager: overlay texture packs with expansion compatibility
- Auth screen expansion picker replacing hardcoded version dropdown

include/pipeline/asset_manager.hpp

@@ -6,6 +6,7 @@
 #include "pipeline/loose_file_reader.hpp"
 #include <memory>
 #include <string>
+#include <vector>
 #include <map>
 #include <mutex>
 
@@ -16,6 +17,8 @@ namespace pipeline {
  * AssetManager - Unified interface for loading WoW assets
  *
  * Reads pre-extracted loose files indexed by manifest.json.
+ * Supports layered manifests: overlay manifests (HD packs, mods)
+ * are checked before the base manifest, with higher priority first.
  * Use the asset_extract tool to extract MPQ archives first.
  * All reads are fully parallel (no serialization mutex needed).
  */
@@ -41,6 +44,26 @@ public:
      */
     bool isInitialized() const { return initialized; }
 
+    /**
+     * Add an overlay manifest (HD packs, mods) checked before the base manifest.
+     * Higher priority overlays are checked first.
+     * @param manifestPath Full path to the overlay's manifest.json
+     * @param priority Priority level (higher = checked first)
+     * @param id Unique identifier for this overlay (e.g. "hd_character")
+     * @return true if overlay loaded successfully
+     */
+    bool addOverlayManifest(const std::string& manifestPath, int priority, const std::string& id);
+
+    /**
+     * Remove a previously added overlay manifest by id.
+     */
+    void removeOverlay(const std::string& id);
+
+    /**
+     * Get list of active overlay IDs.
+     */
+    std::vector<std::string> getOverlayIds() const;
+
     /**
      * Load a BLP texture
      * @param path Virtual path to BLP file (e.g., "Textures\\Minimap\\Background.blp")
@@ -105,10 +128,24 @@ private:
     bool initialized = false;
     std::string dataPath;
 
-    // Loose file backend
+    // Base manifest (loaded from dataPath/manifest.json)
     AssetManifest manifest_;
     LooseFileReader looseReader_;
 
+    // Overlay manifests (HD packs, mods) - sorted by priority descending
+    struct ManifestLayer {
+        AssetManifest manifest;
+        int priority;
+        std::string id;
+    };
+    std::vector<ManifestLayer> overlayLayers_;  // Sorted by priority desc
+
+    /**
+     * Resolve filesystem path checking overlays first, then base manifest.
+     * Returns empty string if not found in any layer.
+     */
+    std::string resolveLayeredPath(const std::string& normalizedPath) const;
+
     mutable std::mutex cacheMutex;
     std::map<std::string, std::shared_ptr<DBCFile>> dbcCache;