Replace the 3-message XK handshake with XX for FSP session establishment.
XX requires no prior knowledge of the peer's static key — the responder's
identity is revealed in msg2, the initiator's in msg3.
Key changes:
- session.rs: XX initiator/responder, post-handshake identity verification
using x-only key comparison (parity-independent for npub compatibility),
negotiation payload in msg2/msg3 (FSP version [0,0], features=0)
- Rekey: switched from XK to XX for FSP rekey handshake
- timeout.rs: suppress msg1 resends when target peer is already promoted,
preventing cross-connection session mismatch from duplicate handshakes
- Test template: discovery backoff 3s and handshake timeout 10s for
faster convergence in integration tests
- Integration test timeouts restored to 45s (ping) and 60s (rekey)
Squashed commits:
- Switch FSP handshake from Noise XK to XX
- Fix integration test convergence by reducing discovery backoff
- Fix cross-connection session mismatch from msg1 resend
- Fix FSP identity verification parity mismatch
Update the default UDP bind port from 4000 to 2121 (decimal) and the
default Ethernet EtherType from 0x88B5 to 0x2121 across all source
code, documentation, configuration templates, test fixtures, and
scripts. Remove references to "IEEE 802 experimental range" since
0x2121 is not in that range.
Johnathan Corgan