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/mod.rs

@@ -390,6 +390,10 @@ pub struct Node {
     /// Pending outbound handshakes by our sender_idx.
     /// Tracks which LinkId corresponds to which session index.
     pending_outbound: HashMap<(TransportId, u32), LinkId>,
+    /// Pending inbound connections awaiting msg3 (XX pattern), keyed by
+    /// (transport_id, our_index). The responder stores the connection here
+    /// after sending msg2 and awaits msg3 to learn the initiator's identity.
+    pending_inbound: HashMap<(TransportId, u32), LinkId>,
 
     // === Rate Limiting ===
     /// Rate limiter for msg1 processing (DoS protection).
@@ -547,6 +551,7 @@ impl Node {
             index_allocator: IndexAllocator::new(),
             peers_by_index: HashMap::new(),
             pending_outbound: HashMap::new(),
+            pending_inbound: HashMap::new(),
             msg1_rate_limiter,
             icmp_rate_limiter: IcmpRateLimiter::new(),
             routing_error_rate_limiter: RoutingErrorRateLimiter::new(),
@@ -656,6 +661,7 @@ impl Node {
             index_allocator: IndexAllocator::new(),
             peers_by_index: HashMap::new(),
             pending_outbound: HashMap::new(),
+            pending_inbound: HashMap::new(),
             msg1_rate_limiter,
             icmp_rate_limiter: IcmpRateLimiter::new(),
             routing_error_rate_limiter: RoutingErrorRateLimiter::new(),