refactor(analysis): DTLS's foundation analysis wraps a DTLS protocol method, and the analysis convention is written down #2622

Owner ruling, given in session and paraphrased: pcapkit.foundation.analysis is a public API layer over protocol internals, so each function in it calls a protocol method and holds no parsing of its own. That is how ip_datagram/tcp_datagram work (#2619): they call IPv4/IPv6/TCP.analyze. The owner asked that the same rule apply to DTLS, and that the convention be recorded.

On main, foundation.analysis.dtls._read_handshake holds the logic itself: it rebuilds the message from the reassembled header and body, calls DTLS.read_handshake, and falls back to a raw DTLSHandshake on ProtocolError. The reassembly Deferred calls it too.

Plan:

• Move that logic into a classmethod on the DTLS protocol. dtls_handshake and the Deferred then call it. Signatures and outputs stay the same.
• Write the convention onto reassembly-integration.rst: the protocol class does the parsing; foundation.analysis and the reassembly Deferred call it; protocol code never imports foundation to do its own work.