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.

extern/lua-5.1.5/src/linit.c

@@ -0,0 +1,38 @@
+/*
+** $Id: linit.c,v 1.14.1.1 2007/12/27 13:02:25 roberto Exp $
+** Initialization of libraries for lua.c
+** See Copyright Notice in lua.h
+*/
+
+
+#define linit_c
+#define LUA_LIB
+
+#include "lua.h"
+
+#include "lualib.h"
+#include "lauxlib.h"
+
+
+static const luaL_Reg lualibs[] = {
+  {"", luaopen_base},
+  {LUA_LOADLIBNAME, luaopen_package},
+  {LUA_TABLIBNAME, luaopen_table},
+  {LUA_IOLIBNAME, luaopen_io},
+  {LUA_OSLIBNAME, luaopen_os},
+  {LUA_STRLIBNAME, luaopen_string},
+  {LUA_MATHLIBNAME, luaopen_math},
+  {LUA_DBLIBNAME, luaopen_debug},
+  {NULL, NULL}
+};
+
+
+LUALIB_API void luaL_openlibs (lua_State *L) {
+  const luaL_Reg *lib = lualibs;
+  for (; lib->func; lib++) {
+    lua_pushcfunction(L, lib->func);
+    lua_pushstring(L, lib->name);
+    lua_call(L, 1, 0);
+  }
+}
+