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/trace-globals.lua

@@ -0,0 +1,38 @@
+-- trace assigments to global variables
+
+do
+ -- a tostring that quotes strings. note the use of the original tostring.
+ local _tostring=tostring
+ local tostring=function(a)
+  if type(a)=="string" then
+   return string.format("%q",a)
+  else
+   return _tostring(a)
+  end
+ end
+
+ local log=function (name,old,new)
+  local t=debug.getinfo(3,"Sl")
+  local line=t.currentline
+  io.write(t.short_src)
+  if line>=0 then io.write(":",line) end
+  io.write(": ",name," is now ",tostring(new)," (was ",tostring(old),")","\n")
+ end
+
+ local g={}
+ local set=function (t,name,value)
+  log(name,g[name],value)
+  g[name]=value
+ end
+ setmetatable(getfenv(),{__index=g,__newindex=set})
+end
+
+-- an example
+
+a=1
+b=2
+a=10
+b=20
+b=nil
+b=200
+print(a,b,c)