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/README

@@ -0,0 +1,37 @@
+This directory contains some useful files and code.
+Unlike the code in ../src, everything here is in the public domain.
+
+If any of the makes fail, you're probably not using the same libraries
+used to build Lua. Set MYLIBS in Makefile accordingly.
+
+all.c
+	Full Lua interpreter in a single file.
+	Do "make one" for a demo.
+
+lua.hpp
+	Lua header files for C++ using 'extern "C"'.
+
+lua.ico
+	A Lua icon for Windows (and web sites: save as favicon.ico).
+	Drawn by hand by Markus Gritsch <gritsch@iue.tuwien.ac.at>.
+
+lua.pc
+	pkg-config data for Lua
+
+luavs.bat
+	Script to build Lua under "Visual Studio .NET Command Prompt".
+	Run it from the toplevel as etc\luavs.bat.
+
+min.c
+	A minimal Lua interpreter.
+	Good for learning and for starting your own.
+	Do "make min" for a demo.
+
+noparser.c
+	Linking with noparser.o avoids loading the parsing modules in lualib.a.
+	Do "make noparser" for a demo.
+
+strict.lua
+	Traps uses of undeclared global variables.
+	Do "make strict" for a demo.
+