Goroutine leaks and silent TLS config fallback in arke Serve

A few issues in the server's Serve path:

• listener() silently falls back to cleartext when tlsConfig returns an error, even though the operator asked for TLS. The error should be surfaced instead.
• serveErrChan is unbuffered and the non-OpError branch does not return, leaving a sender blocked on shutdown and leaking the goroutine.
• The signal-handler goroutine does not exit on context cancel and signal.Notify is never stopped, so goroutines accumulate across repeated Serve calls.

Start in the Serve path, tracing listener(), serveErrChan, the non-OpError branch, and the signal-handler goroutine using signal.Notify. Confirm each shutdown path and verify that TLS errors are surfaced, error sends cannot block, and repeated Serve calls do not accumulate goroutines.