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/README

@@ -0,0 +1,26 @@
+These are simple tests for Lua.  Some of them contain useful code.
+They are meant to be run to make sure Lua is built correctly and also
+to be read, to see how Lua programs look.
+
+Here is a one-line summary of each program:
+
+   bisect.lua		bisection method for solving non-linear equations
+   cf.lua		temperature conversion table (celsius to farenheit)
+   echo.lua             echo command line arguments
+   env.lua              environment variables as automatic global variables
+   factorial.lua	factorial without recursion
+   fib.lua		fibonacci function with cache
+   fibfor.lua		fibonacci numbers with coroutines and generators
+   globals.lua		report global variable usage
+   hello.lua		the first program in every language
+   life.lua		Conway's Game of Life
+   luac.lua	 	bare-bones luac
+   printf.lua		an implementation of printf
+   readonly.lua		make global variables readonly
+   sieve.lua		the sieve of of Eratosthenes programmed with coroutines
+   sort.lua		two implementations of a sort function
+   table.lua		make table, grouping all data for the same item
+   trace-calls.lua	trace calls
+   trace-globals.lua	trace assigments to global variables
+   xd.lua		hex dump
+