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/doc/manual.css

@@ -0,0 +1,24 @@
+h3 code {
+	font-family: inherit ;
+	font-size: inherit ;
+}
+
+pre, code {
+	font-size: 12pt ;
+}
+
+span.apii {
+	float: right ;
+	font-family: inherit ;
+	font-style: normal ;
+	font-size: small ;
+	color: gray ;
+}
+
+p+h1, ul+h1 {
+	padding-top: 0.4em ;
+	padding-bottom: 0.4em ;
+	padding-left: 30px ;
+	margin-left: -30px ;
+	background-color: #E0E0FF ;
+}