Live verification on a Windows VM surfaced a crash loop: TcpFrameServer.EnsureListening
assigned _listener = new TcpListener(...) BEFORE calling Start(). When Start() throws —
e.g. the port is in a Windows excluded/reserved range (WSAEACCES) or already in use — the
field was left non-null-but-unstarted, so the `if (_listener is not null) return` guard
permanently skipped re-Start() and every subsequent AcceptTcpClientAsync() threw the
misleading InvalidOperationException "Not listening" → 20 failures → exit 2 → NSSM restart
→ loop. Now _listener is assigned only after Start() succeeds, so a transient bind failure
is retried and a permanent one surfaces the real bind error each iteration. Adds a
regression test that forces a bind conflict and asserts the SocketException persists.
Joseph Doherty