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/luac.lua

@@ -0,0 +1,7 @@
+-- bare-bones luac in Lua
+-- usage: lua luac.lua file.lua
+
+assert(arg[1]~=nil and arg[2]==nil,"usage: lua luac.lua file.lua")
+f=assert(io.open("luac.out","wb"))
+assert(f:write(string.dump(assert(loadfile(arg[1])))))
+assert(f:close())