Add DTLS close state predicates

[algesten]

Summary:

• Add public Dtls::is_closing() and Dtls::is_closed() predicates with drain-aware shutdown semantics.
• Keep DTLS 1.2 closed reporting false until queued close_notify packets and CloseNotify events are polled out.
• Cover DTLS 1.2 reciprocal close and DTLS 1.3 half-close transitions in edge tests.

Closes #155