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/hd_pack_manager.hpp

@@ -0,0 +1,97 @@
+#pragma once
+
+#include <cstdint>
+#include <string>
+#include <vector>
+#include <unordered_map>
+
+namespace wowee {
+namespace pipeline {
+
+class AssetManager;
+
+/**
+ * Metadata for a single HD texture pack on disk.
+ *
+ * Each pack lives in Data/hd/<packDir>/ and contains:
+ *   pack.json      - metadata (id, name, group, compatible expansions, size)
+ *   manifest.json  - standard asset manifest with HD override textures
+ *   assets/        - the actual HD files
+ */
+struct HDPack {
+    std::string id;                         // Unique identifier (e.g. "character_hd")
+    std::string name;                       // Human-readable name
+    std::string group;                      // Grouping label (e.g. "Character", "Terrain")
+    std::vector<std::string> expansions;    // Compatible expansion IDs
+    uint32_t totalSizeMB = 0;              // Approximate total size on disk
+    std::string manifestPath;              // Full path to manifest.json
+    std::string packDir;                   // Full path to pack directory
+    bool enabled = false;                  // User-toggled enable state
+};
+
+/**
+ * HDPackManager - discovers, manages, and wires HD texture packs.
+ *
+ * Scans Data/hd/ subdirectories for pack.json files. Each pack can be
+ * enabled/disabled via setPackEnabled(). Enabled packs are wired into
+ * AssetManager as high-priority overlay manifests so that HD textures
+ * override the base expansion assets transparently.
+ */
+class HDPackManager {
+public:
+    HDPackManager() = default;
+
+    /**
+     * Scan the HD root directory for available packs.
+     * @param hdRootPath Path to Data/hd/ directory
+     */
+    void initialize(const std::string& hdRootPath);
+
+    /**
+     * Get all discovered packs.
+     */
+    const std::vector<HDPack>& getAllPacks() const { return packs_; }
+
+    /**
+     * Get packs compatible with a specific expansion.
+     */
+    std::vector<const HDPack*> getPacksForExpansion(const std::string& expansionId) const;
+
+    /**
+     * Enable or disable a pack. Persists state in enabledPacks_ map.
+     */
+    void setPackEnabled(const std::string& packId, bool enabled);
+
+    /**
+     * Check if a pack is enabled.
+     */
+    bool isPackEnabled(const std::string& packId) const;
+
+    /**
+     * Apply enabled packs as overlays to the asset manager.
+     * Removes previously applied overlays and re-adds enabled ones.
+     */
+    void applyToAssetManager(AssetManager* assetManager, const std::string& expansionId);
+
+    /**
+     * Save enabled pack state to a settings file.
+     */
+    void saveSettings(const std::string& settingsPath) const;
+
+    /**
+     * Load enabled pack state from a settings file.
+     */
+    void loadSettings(const std::string& settingsPath);
+
+private:
+    std::vector<HDPack> packs_;
+    std::unordered_map<std::string, bool> enabledState_;  // packId → enabled
+
+    // Overlay IDs currently applied to AssetManager (for removal on re-apply)
+    std::vector<std::string> appliedOverlayIds_;
+
+    static constexpr int HD_OVERLAY_PRIORITY_BASE = 100;  // High priority, above expansion base
+};
+
+} // namespace pipeline
+} // namespace wowee