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/etc/noparser.c

@@ -0,0 +1,50 @@
+/*
+* The code below can be used to make a Lua core that does not contain the
+* parsing modules (lcode, llex, lparser), which represent 35% of the total core.
+* You'll only be able to load binary files and strings, precompiled with luac.
+* (Of course, you'll have to build luac with the original parsing modules!)
+*
+* To use this module, simply compile it ("make noparser" does that) and list
+* its object file before the Lua libraries. The linker should then not load
+* the parsing modules. To try it, do "make luab".
+*
+* If you also want to avoid the dump module (ldump.o), define NODUMP.
+* #define NODUMP
+*/
+
+#define LUA_CORE
+
+#include "llex.h"
+#include "lparser.h"
+#include "lzio.h"
+
+LUAI_FUNC void luaX_init (lua_State *L) {
+  UNUSED(L);
+}
+
+LUAI_FUNC Proto *luaY_parser (lua_State *L, ZIO *z, Mbuffer *buff, const char *name) {
+  UNUSED(z);
+  UNUSED(buff);
+  UNUSED(name);
+  lua_pushliteral(L,"parser not loaded");
+  lua_error(L);
+  return NULL;
+}
+
+#ifdef NODUMP
+#include "lundump.h"
+
+LUAI_FUNC int luaU_dump (lua_State* L, const Proto* f, lua_Writer w, void* data, int strip) {
+  UNUSED(f);
+  UNUSED(w);
+  UNUSED(data);
+  UNUSED(strip);
+#if 1
+  UNUSED(L);
+  return 0;
+#else
+  lua_pushliteral(L,"dumper not loaded");
+  lua_error(L);
+#endif
+}
+#endif