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/lvm.h

@@ -0,0 +1,36 @@
+/*
+** $Id: lvm.h,v 2.5.1.1 2007/12/27 13:02:25 roberto Exp $
+** Lua virtual machine
+** See Copyright Notice in lua.h
+*/
+
+#ifndef lvm_h
+#define lvm_h
+
+
+#include "ldo.h"
+#include "lobject.h"
+#include "ltm.h"
+
+
+#define tostring(L,o) ((ttype(o) == LUA_TSTRING) || (luaV_tostring(L, o)))
+
+#define tonumber(o,n)	(ttype(o) == LUA_TNUMBER || \
+                         (((o) = luaV_tonumber(o,n)) != NULL))
+
+#define equalobj(L,o1,o2) \
+	(ttype(o1) == ttype(o2) && luaV_equalval(L, o1, o2))
+
+
+LUAI_FUNC int luaV_lessthan (lua_State *L, const TValue *l, const TValue *r);
+LUAI_FUNC int luaV_equalval (lua_State *L, const TValue *t1, const TValue *t2);
+LUAI_FUNC const TValue *luaV_tonumber (const TValue *obj, TValue *n);
+LUAI_FUNC int luaV_tostring (lua_State *L, StkId obj);
+LUAI_FUNC void luaV_gettable (lua_State *L, const TValue *t, TValue *key,
+                                            StkId val);
+LUAI_FUNC void luaV_settable (lua_State *L, const TValue *t, TValue *key,
+                                            StkId val);
+LUAI_FUNC void luaV_execute (lua_State *L, int nexeccalls);
+LUAI_FUNC void luaV_concat (lua_State *L, int total, int last);
+
+#endif