feat: add Lua 5.1 addon system with .toc loader and /run command

Foundation for WoW-compatible addon support:

- Vendor Lua 5.1.5 source as a static library (extern/lua-5.1.5)
- TocParser: parses .toc files (## directives + file lists)
- LuaEngine: Lua 5.1 VM with sandboxed stdlib (no io/os/debug),
  WoW-compatible print() that outputs to chat, GetTime() stub
- AddonManager: scans Data/interface/AddOns/ for .toc files,
  loads .lua files on world entry, skips LoadOnDemand addons
- /run <code> slash command for inline Lua execution
- HelloWorld test addon that prints to chat on load

Integration: AddonManager initialized after asset manager, addons
loaded once on first world entry, reset on logout. XML frame
parsing is deferred to a future step.

include/addons/lua_engine.hpp

@@ -0,0 +1,37 @@
+#pragma once
+
+#include <string>
+
+struct lua_State;
+
+namespace wowee::game { class GameHandler; }
+
+namespace wowee::addons {
+
+class LuaEngine {
+public:
+    LuaEngine();
+    ~LuaEngine();
+
+    LuaEngine(const LuaEngine&) = delete;
+    LuaEngine& operator=(const LuaEngine&) = delete;
+
+    bool initialize();
+    void shutdown();
+
+    bool executeFile(const std::string& path);
+    bool executeString(const std::string& code);
+
+    void setGameHandler(game::GameHandler* handler);
+
+    lua_State* getState() { return L_; }
+    bool isInitialized() const { return L_ != nullptr; }
+
+private:
+    lua_State* L_ = nullptr;
+    game::GameHandler* gameHandler_ = nullptr;
+
+    void registerCoreAPI();
+};
+
+} // namespace wowee::addons