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/lmem.h

@@ -0,0 +1,49 @@
+/*
+** $Id: lmem.h,v 1.31.1.1 2007/12/27 13:02:25 roberto Exp $
+** Interface to Memory Manager
+** See Copyright Notice in lua.h
+*/
+
+#ifndef lmem_h
+#define lmem_h
+
+
+#include <stddef.h>
+
+#include "llimits.h"
+#include "lua.h"
+
+#define MEMERRMSG	"not enough memory"
+
+
+#define luaM_reallocv(L,b,on,n,e) \
+	((cast(size_t, (n)+1) <= MAX_SIZET/(e)) ?  /* +1 to avoid warnings */ \
+		luaM_realloc_(L, (b), (on)*(e), (n)*(e)) : \
+		luaM_toobig(L))
+
+#define luaM_freemem(L, b, s)	luaM_realloc_(L, (b), (s), 0)
+#define luaM_free(L, b)		luaM_realloc_(L, (b), sizeof(*(b)), 0)
+#define luaM_freearray(L, b, n, t)   luaM_reallocv(L, (b), n, 0, sizeof(t))
+
+#define luaM_malloc(L,t)	luaM_realloc_(L, NULL, 0, (t))
+#define luaM_new(L,t)		cast(t *, luaM_malloc(L, sizeof(t)))
+#define luaM_newvector(L,n,t) \
+		cast(t *, luaM_reallocv(L, NULL, 0, n, sizeof(t)))
+
+#define luaM_growvector(L,v,nelems,size,t,limit,e) \
+          if ((nelems)+1 > (size)) \
+            ((v)=cast(t *, luaM_growaux_(L,v,&(size),sizeof(t),limit,e)))
+
+#define luaM_reallocvector(L, v,oldn,n,t) \
+   ((v)=cast(t *, luaM_reallocv(L, v, oldn, n, sizeof(t))))
+
+
+LUAI_FUNC void *luaM_realloc_ (lua_State *L, void *block, size_t oldsize,
+                                                          size_t size);
+LUAI_FUNC void *luaM_toobig (lua_State *L);
+LUAI_FUNC void *luaM_growaux_ (lua_State *L, void *block, int *size,
+                               size_t size_elem, int limit,
+                               const char *errormsg);
+
+#endif
+