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/luavs.bat

@@ -0,0 +1,28 @@
+@rem Script to build Lua under "Visual Studio .NET Command Prompt".
+@rem Do not run from this directory; run it from the toplevel: etc\luavs.bat .
+@rem It creates lua51.dll, lua51.lib, lua.exe, and luac.exe in src.
+@rem (contributed by David Manura and Mike Pall)
+
+@setlocal
+@set MYCOMPILE=cl /nologo /MD /O2 /W3 /c /D_CRT_SECURE_NO_DEPRECATE
+@set MYLINK=link /nologo
+@set MYMT=mt /nologo
+
+cd src
+%MYCOMPILE% /DLUA_BUILD_AS_DLL l*.c
+del lua.obj luac.obj
+%MYLINK% /DLL /out:lua51.dll l*.obj
+if exist lua51.dll.manifest^
+  %MYMT% -manifest lua51.dll.manifest -outputresource:lua51.dll;2
+%MYCOMPILE% /DLUA_BUILD_AS_DLL lua.c
+%MYLINK% /out:lua.exe lua.obj lua51.lib
+if exist lua.exe.manifest^
+  %MYMT% -manifest lua.exe.manifest -outputresource:lua.exe
+%MYCOMPILE% l*.c print.c
+del lua.obj linit.obj lbaselib.obj ldblib.obj liolib.obj lmathlib.obj^
+    loslib.obj ltablib.obj lstrlib.obj loadlib.obj
+%MYLINK% /out:luac.exe *.obj
+if exist luac.exe.manifest^
+  %MYMT% -manifest luac.exe.manifest -outputresource:luac.exe
+del *.obj *.manifest
+cd ..