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/trace-calls.lua

@@ -0,0 +1,32 @@
+-- trace calls
+-- example: lua -ltrace-calls bisect.lua
+
+local level=0
+
+local function hook(event)
+ local t=debug.getinfo(3)
+ io.write(level," >>> ",string.rep(" ",level))
+ if t~=nil and t.currentline>=0 then io.write(t.short_src,":",t.currentline," ") end
+ t=debug.getinfo(2)
+ if event=="call" then
+  level=level+1
+ else
+  level=level-1 if level<0 then level=0 end
+ end
+ if t.what=="main" then
+  if event=="call" then
+   io.write("begin ",t.short_src)
+  else
+   io.write("end ",t.short_src)
+  end
+ elseif t.what=="Lua" then
+-- table.foreach(t,print)
+  io.write(event," ",t.name or "(Lua)"," <",t.linedefined,":",t.short_src,">")
+ else
+ io.write(event," ",t.name or "(C)"," [",t.what,"] ")
+ end
+ io.write("\n")
+end
+
+debug.sethook(hook,"cr")
+level=0