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.

src/ui/game_screen.cpp

@@ -2,6 +2,7 @@
 #include "rendering/character_preview.hpp"
 #include "rendering/vk_context.hpp"
 #include "core/application.hpp"
+#include "addons/addon_manager.hpp"
 #include "core/coordinates.hpp"
 #include "core/spawn_presets.hpp"
 #include "core/input.hpp"
@@ -2629,7 +2630,7 @@ void GameScreen::renderChatWindow(game::GameHandler& gameHandler) {
                     "/p", "/party", "/petaggressive", "/petattack", "/petdefensive",
                     "/petdismiss", "/petfollow", "/pethalt", "/petpassive", "/petstay",
                     "/played", "/pvp",
-                    "/r", "/raid", "/raidinfo", "/raidwarning", "/random", "/reply", "/roll",
+                    "/r", "/raid", "/raidinfo", "/raidwarning", "/random", "/reply", "/roll", "/run",
                     "/s", "/say", "/screenshot", "/setloot", "/shout", "/sit", "/stand",
                     "/startattack", "/stopattack", "/stopcasting", "/stopfollow", "/stopmacro",
                     "/t", "/target", "/threat", "/time", "/trade",
@@ -5993,6 +5994,19 @@ void GameScreen::sendChatMessage(game::GameHandler& gameHandler) {
             std::string cmdLower = cmd;
             for (char& c : cmdLower) c = std::tolower(c);
 
+            // /run <lua code> — execute Lua script via addon system
+            if ((cmdLower == "run" || cmdLower == "script") && spacePos != std::string::npos) {
+                std::string luaCode = command.substr(spacePos + 1);
+                auto* am = core::Application::getInstance().getAddonManager();
+                if (am) {
+                    am->runScript(luaCode);
+                } else {
+                    gameHandler.addUIError("Addon system not initialized.");
+                }
+                chatInputBuffer[0] = '\0';
+                return;
+            }
+
             // Special commands
             if (cmdLower == "logout") {
                 core::Application::getInstance().logoutToLogin();