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/cf.lua

@@ -0,0 +1,16 @@
+-- temperature conversion table (celsius to farenheit)
+
+for c0=-20,50-1,10 do
+	io.write("C ")
+	for c=c0,c0+10-1 do
+		io.write(string.format("%3.0f ",c))
+	end
+	io.write("\n")
+	
+	io.write("F ")
+	for c=c0,c0+10-1 do
+		f=(9/5)*c+32
+		io.write(string.format("%3.0f ",f))
+	end
+	io.write("\n\n")
+end