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/factorial.lua

@@ -0,0 +1,32 @@
+-- function closures are powerful
+
+-- traditional fixed-point operator from functional programming
+Y = function (g)
+      local a = function (f) return f(f) end
+      return a(function (f)
+                 return g(function (x)
+                             local c=f(f)
+                             return c(x)
+                           end)
+               end)
+end
+
+
+-- factorial without recursion
+F = function (f)
+      return function (n)
+               if n == 0 then return 1
+               else return n*f(n-1) end
+             end
+    end
+
+factorial = Y(F)   -- factorial is the fixed point of F
+
+-- now test it
+function test(x)
+	io.write(x,"! = ",factorial(x),"\n")
+end
+
+for n=0,16 do
+	test(n)
+end