Switch FMP handshake from Noise IK to XX with version negotiation

Replace the 2-message IK handshake with a 3-message XX handshake for
FMP link establishment. XX requires no prior knowledge of the peer's
static key — both identities are revealed during the handshake
(responder in msg2, initiator in msg3). This is the foundation for
the forklift upgrade that enables rolling protocol upgrades.

Changes:
- Noise XX state machine alongside IK/XK (8 unit tests)
- Protocol negotiation payload codec: format byte, packed version
  min/max, 64-bit feature bitfield, TLV extensions (11 unit tests)
- FMP wire format version 0→1, msg3 header/builder, TCP stream framing
- FMP handshake switched to XX: PeerConnection 3-message flow,
  handle_msg1 simplified (no identity), handle_msg2 sends msg3 and
  promotes initiator, new handle_msg3 promotes responder with
  restart/rekey/cross-connection detection
- Rekey handshake switched to XX with negotiation payload hash chain
  fix (decrypt-and-discard in complete_rekey_msg2/msg3)
- Negotiation payload in msg2/msg3 (FMP version [1,1], features=0)
- Debug logging for handshake promotion paths
- Integration test convergence timeouts adjusted for extra round-trip

Squashed commits:
- Add Noise XX state machine alongside IK/XK
- Add protocol negotiation payload codec
- FMP wire format prep: version 1, msg3 header support
- Switch FMP handshake from Noise IK to XX
- Increase convergence timeouts for XX 3-message handshake
- Fix negotiation hash chain desync in rekey handshake

src/node/tests/mod.rs

@@ -64,11 +64,14 @@ pub(super) fn make_completed_connection(
     let mut resp_epoch = [0u8; 8];
     rand::Rng::fill_bytes(&mut rand::rng(), &mut resp_epoch);
     let msg2 = resp_conn
-        .receive_handshake_init(peer_keypair, resp_epoch, &msg1, current_time_ms)
+        .receive_handshake_init(peer_keypair, resp_epoch, &msg1, None, current_time_ms)
         .unwrap();
 
-    // Complete initiator handshake
-    conn.complete_handshake(&msg2, current_time_ms).unwrap();
+    // Complete initiator handshake (XX: generates msg3)
+    let (msg3, _neg) = conn.complete_handshake(&msg2, None, current_time_ms).unwrap();
+
+    // Complete responder handshake (XX: processes msg3)
+    resp_conn.complete_handshake_msg3(&msg3, current_time_ms).unwrap();
 
     // Set indices and transport info
     let our_index = node.index_allocator.allocate().unwrap();