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.

Data/interface/AddOns/HelloWorld/HelloWorld.lua

@@ -0,0 +1,3 @@
+-- HelloWorld addon — test the WoWee addon system
+print("|cff00ff00[HelloWorld]|r Addon loaded! Lua 5.1 is working.")
+print("|cff00ff00[HelloWorld]|r GetTime() = " .. string.format("%.2f", GetTime()) .. " seconds")

Data/interface/AddOns/HelloWorld/HelloWorld.toc

@@ -0,0 +1,4 @@
+## Interface: 30300
+## Title: Hello World
+## Notes: Test addon for the WoWee addon system
+HelloWorld.lua