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/etc/strict.lua

@@ -0,0 +1,41 @@
+--
+-- strict.lua
+-- checks uses of undeclared global variables
+-- All global variables must be 'declared' through a regular assignment
+-- (even assigning nil will do) in a main chunk before being used
+-- anywhere or assigned to inside a function.
+--
+
+local getinfo, error, rawset, rawget = debug.getinfo, error, rawset, rawget
+
+local mt = getmetatable(_G)
+if mt == nil then
+  mt = {}
+  setmetatable(_G, mt)
+end
+
+mt.__declared = {}
+
+local function what ()
+  local d = getinfo(3, "S")
+  return d and d.what or "C"
+end
+
+mt.__newindex = function (t, n, v)
+  if not mt.__declared[n] then
+    local w = what()
+    if w ~= "main" and w ~= "C" then
+      error("assign to undeclared variable '"..n.."'", 2)
+    end
+    mt.__declared[n] = true
+  end
+  rawset(t, n, v)
+end
+  
+mt.__index = function (t, n)
+  if not mt.__declared[n] and what() ~= "C" then
+    error("variable '"..n.."' is not declared", 2)
+  end
+  return rawget(t, n)
+end
+