p2p: implement BIP434 peer feature negotiation #1634

Goal

Implement BIP434 Peer Feature Negotiation in crates/p2p so bitcoin-rs can negotiate independent P2P extensions with compatible peers. Bitcoin Core implements BIP434 (PR #35221); BIP332 depends on it.

Implementation

• Raise the advertised P2P protocol version to 70017 when BIP434 is supported, preserving normal compatibility with older peers.
• Add the feature message codec: featureid length 4–80, featuredata length ≤512, minimal CompactSize encoding, exact payload consumption, and bounded parsing. Support the BIP324 v2 short message ID 37 for pre-verack messages.
• Send and accept feature announcements after version and before verack, within the existing handshake FSM and peer capability state. Never send them to peers advertising a protocol version below 70017.
• Ignore well-formed unknown feature IDs. Make enabled features available to existing peer dispatch/relay logic without introducing an independent handshake/peer-state owner.
• Keep existing BIP339 wtxidrelay, BIP155 sendaddrv2, BIP324 transport, and older-peer fallback working.

Acceptance criteria

• Exchange BIP434 feature announcements with a Core implementation supporting BIP434; verify v1 and v2 transports.
• Cover supported/unsupported feature IDs, bounds, malformed/trailing bytes, duplicate announcements, version gating, and pre-/post-verack ordering.
• Verify normal 70016 and older-peer sessions establish without negotiation or disconnects.
• Update crates/p2p/src/compat.rs and docs/policies/p2p-compatibility.md with the precise version/message change.

Related

• BIP332 Stale Tip Relay requires BIP434.
• Current P2P code advertises 70016 in crates/p2p/src/wire.rs and has no typed feature message.
• This is a P2P-only extension; block validity and chain selection are unchanged.