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/lstring.h

@@ -0,0 +1,31 @@
+/*
+** $Id: lstring.h,v 1.43.1.1 2007/12/27 13:02:25 roberto Exp $
+** String table (keep all strings handled by Lua)
+** See Copyright Notice in lua.h
+*/
+
+#ifndef lstring_h
+#define lstring_h
+
+
+#include "lgc.h"
+#include "lobject.h"
+#include "lstate.h"
+
+
+#define sizestring(s)	(sizeof(union TString)+((s)->len+1)*sizeof(char))
+
+#define sizeudata(u)	(sizeof(union Udata)+(u)->len)
+
+#define luaS_new(L, s)	(luaS_newlstr(L, s, strlen(s)))
+#define luaS_newliteral(L, s)	(luaS_newlstr(L, "" s, \
+                                 (sizeof(s)/sizeof(char))-1))
+
+#define luaS_fix(s)	l_setbit((s)->tsv.marked, FIXEDBIT)
+
+LUAI_FUNC void luaS_resize (lua_State *L, int newsize);
+LUAI_FUNC Udata *luaS_newudata (lua_State *L, size_t s, Table *e);
+LUAI_FUNC TString *luaS_newlstr (lua_State *L, const char *str, size_t l);
+
+
+#endif