feat(napi): implement serve() function with WraithServer, WraithServerStream, and ConnectionInfo

Expose NAPI serve() per ADR-016. WraithServer provides close() and
onConnection(callback) for receiving SSH channel streams from
incoming connections. Each connection produces a WraithServerStream
(Duplex-like read/write/close) with ConnectionInfo (remoteAddr,
transportKind). Supports TCP transport with optional authorizedKeys
and certAuthority auth. TLS and iroh transports return helpful errors
indicating future support.

crates/wraith-napi/src/lib.rs

@@ -2,4 +2,5 @@
 #[macro_use]
 extern crate napi_derive;
 
-mod connect;
+mod connect;
+mod serve;