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/ldo.h

@@ -0,0 +1,57 @@
+/*
+** $Id: ldo.h,v 2.7.1.1 2007/12/27 13:02:25 roberto Exp $
+** Stack and Call structure of Lua
+** See Copyright Notice in lua.h
+*/
+
+#ifndef ldo_h
+#define ldo_h
+
+
+#include "lobject.h"
+#include "lstate.h"
+#include "lzio.h"
+
+
+#define luaD_checkstack(L,n)	\
+  if ((char *)L->stack_last - (char *)L->top <= (n)*(int)sizeof(TValue)) \
+    luaD_growstack(L, n); \
+  else condhardstacktests(luaD_reallocstack(L, L->stacksize - EXTRA_STACK - 1));
+
+
+#define incr_top(L) {luaD_checkstack(L,1); L->top++;}
+
+#define savestack(L,p)		((char *)(p) - (char *)L->stack)
+#define restorestack(L,n)	((TValue *)((char *)L->stack + (n)))
+
+#define saveci(L,p)		((char *)(p) - (char *)L->base_ci)
+#define restoreci(L,n)		((CallInfo *)((char *)L->base_ci + (n)))
+
+
+/* results from luaD_precall */
+#define PCRLUA		0	/* initiated a call to a Lua function */
+#define PCRC		1	/* did a call to a C function */
+#define PCRYIELD	2	/* C funtion yielded */
+
+
+/* type of protected functions, to be ran by `runprotected' */
+typedef void (*Pfunc) (lua_State *L, void *ud);
+
+LUAI_FUNC int luaD_protectedparser (lua_State *L, ZIO *z, const char *name);
+LUAI_FUNC void luaD_callhook (lua_State *L, int event, int line);
+LUAI_FUNC int luaD_precall (lua_State *L, StkId func, int nresults);
+LUAI_FUNC void luaD_call (lua_State *L, StkId func, int nResults);
+LUAI_FUNC int luaD_pcall (lua_State *L, Pfunc func, void *u,
+                                        ptrdiff_t oldtop, ptrdiff_t ef);
+LUAI_FUNC int luaD_poscall (lua_State *L, StkId firstResult);
+LUAI_FUNC void luaD_reallocCI (lua_State *L, int newsize);
+LUAI_FUNC void luaD_reallocstack (lua_State *L, int newsize);
+LUAI_FUNC void luaD_growstack (lua_State *L, int n);
+
+LUAI_FUNC void luaD_throw (lua_State *L, int errcode);
+LUAI_FUNC int luaD_rawrunprotected (lua_State *L, Pfunc f, void *ud);
+
+LUAI_FUNC void luaD_seterrorobj (lua_State *L, int errcode, StkId oldtop);
+
+#endif
+