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/README

@@ -0,0 +1,37 @@
+README for Lua 5.1
+
+See INSTALL for installation instructions.
+See HISTORY for a summary of changes since the last released version.
+
+* What is Lua?
+  ------------
+  Lua is a powerful, light-weight programming language designed for extending
+  applications. Lua is also frequently used as a general-purpose, stand-alone
+  language. Lua is free software.
+
+  For complete information, visit Lua's web site at http://www.lua.org/ .
+  For an executive summary, see http://www.lua.org/about.html .
+
+  Lua has been used in many different projects around the world.
+  For a short list, see http://www.lua.org/uses.html .
+
+* Availability
+  ------------
+  Lua is freely available for both academic and commercial purposes.
+  See COPYRIGHT and http://www.lua.org/license.html for details.
+  Lua can be downloaded at http://www.lua.org/download.html .
+
+* Installation
+  ------------
+  Lua is implemented in pure ANSI C, and compiles unmodified in all known
+  platforms that have an ANSI C compiler. In most Unix-like platforms, simply
+  do "make" with a suitable target. See INSTALL for detailed instructions.
+
+* Origin
+  ------
+  Lua is developed at Lua.org, a laboratory of the Department of Computer
+  Science of PUC-Rio (the Pontifical Catholic University of Rio de Janeiro
+  in Brazil).
+  For more information about the authors, see http://www.lua.org/authors.html .
+
+(end of README)