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/fib.lua

@@ -0,0 +1,40 @@
+-- fibonacci function with cache
+
+-- very inefficient fibonacci function
+function fib(n)
+	N=N+1
+	if n<2 then
+		return n
+	else
+		return fib(n-1)+fib(n-2)
+	end
+end
+
+-- a general-purpose value cache
+function cache(f)
+	local c={}
+	return function (x)
+		local y=c[x]
+		if not y then
+			y=f(x)
+			c[x]=y
+		end
+		return y
+	end
+end
+
+-- run and time it
+function test(s,f)
+	N=0
+	local c=os.clock()
+	local v=f(n)
+	local t=os.clock()-c
+	print(s,n,v,t,N)
+end
+
+n=arg[1] or 24		-- for other values, do lua fib.lua XX
+n=tonumber(n)
+print("","n","value","time","evals")
+test("plain",fib)
+fib=cache(fib)
+test("cached",fib)