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/etc/lua.pc

@@ -0,0 +1,31 @@
+# lua.pc -- pkg-config data for Lua
+
+# vars from install Makefile
+
+# grep '^V=' ../Makefile
+V= 5.1
+# grep '^R=' ../Makefile
+R= 5.1.5
+
+# grep '^INSTALL_.*=' ../Makefile | sed 's/INSTALL_TOP/prefix/'
+prefix= /usr/local
+INSTALL_BIN= ${prefix}/bin
+INSTALL_INC= ${prefix}/include
+INSTALL_LIB= ${prefix}/lib
+INSTALL_MAN= ${prefix}/man/man1
+INSTALL_LMOD= ${prefix}/share/lua/${V}
+INSTALL_CMOD= ${prefix}/lib/lua/${V}
+
+# canonical vars
+exec_prefix=${prefix}
+libdir=${exec_prefix}/lib
+includedir=${prefix}/include
+
+Name: Lua
+Description: An Extensible Extension Language
+Version: ${R}
+Requires: 
+Libs: -L${libdir} -llua -lm
+Cflags: -I${includedir}
+
+# (end of lua.pc)