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/sieve.lua

@@ -0,0 +1,29 @@
+-- the sieve of of Eratosthenes programmed with coroutines
+-- typical usage: lua -e N=1000 sieve.lua | column
+
+-- generate all the numbers from 2 to n
+function gen (n)
+  return coroutine.wrap(function ()
+    for i=2,n do coroutine.yield(i) end
+  end)
+end
+
+-- filter the numbers generated by `g', removing multiples of `p'
+function filter (p, g)
+  return coroutine.wrap(function ()
+    while 1 do
+      local n = g()
+      if n == nil then return end
+      if math.mod(n, p) ~= 0 then coroutine.yield(n) end
+    end
+  end)
+end
+
+N=N or 1000		-- from command line
+x = gen(N)		-- generate primes up to N
+while 1 do
+  local n = x()		-- pick a number until done
+  if n == nil then break end
+  print(n)		-- must be a prime number
+  x = filter(n, x)	-- now remove its multiples
+end