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/test/readonly.lua

@@ -0,0 +1,12 @@
+-- make global variables readonly
+
+local f=function (t,i) error("cannot redefine global variable `"..i.."'",2) end
+local g={}
+local G=getfenv()
+setmetatable(g,{__index=G,__newindex=f})
+setfenv(1,g)
+
+-- an example
+rawset(g,"x",3)
+x=2
+y=1	-- cannot redefine `y'