Add periodic Noise rekey with fresh DH for forward secrecy (FMP + FSP)

Implement periodic full rekey at both protocol layers using fresh DH
key exchanges. Uses the existing K-bit flag (FLAG_KEY_EPOCH /
FSP_FLAG_K) to coordinate cutover between peers.

FMP layer (IK pattern):
- ActivePeer gains rekey state: pending/previous sessions, K-bit epoch
  tracking, drain window, dampening timer
- Handshake state stored on ActivePeer with msg1 sent on existing link
- Encrypted frame handler detects K-bit flips, promotes pending
  sessions, falls back to previous session during drain
- Handshake handlers distinguish rekey from new connections using
  addr_to_link lookup with identity-based fallback
- Free all session indices (current, rekey, pending, previous) on
  peer removal

FSP layer (XK pattern):
- SessionEntry gains parallel rekey fields with XK-specific state
  for the 3-message handshake
- Route availability check before FSP rekey initiation
- Encrypted session handler adds K-bit flip detection and dual-session
  decrypt fallback
- SessionSetup/Ack/Msg3 handlers extended for rekey paths

Defense-in-depth:
- Consecutive decryption failure detector (threshold=20) triggers
  forced peer removal instead of waiting for link-dead timeout
- Identity-based rekey detection as fallback when addr_to_link
  doesn't match (e.g., TCP ephemeral ports)

Configuration: RekeyConfig with enabled flag, after_secs (default 120),
and after_messages (default 65536) thresholds.

Logging: info for successful K-bit cutover completions, warn for
failures, debug for intermediate handshake steps, trace for routine
operations (resends, drain cleanup).

Rekey lifecycle:
1. Timer/counter fires -> initiator starts new handshake
2. Old session continues handling traffic during handshake
3. Handshake completes -> initiator cuts over, flips K-bit
4. Responder sees flipped K-bit -> promotes new session
5. Both keep old session for 10s drain window
6. After drain, old session discarded

Integration test: Docker-based multi-phase test exercising both FMP
and FSP rekey with aggressive timers (35s). Verifies connectivity
across all 20 directed pairs survives two consecutive rekey cycles.
Includes rekey topology, docker-compose profile, and CI matrix entry.

Increase ping test convergence wait from 3s to 5s for CI reliability.

src/node/mod.rs

@@ -35,7 +35,7 @@ use crate::transport::ethernet::EthernetTransport;
 use crate::tree::TreeState;
 use crate::upper::icmp_rate_limit::IcmpRateLimiter;
 use crate::upper::tun::{TunError, TunOutboundRx, TunState, TunTx};
-use self::wire::{build_encrypted, build_established_header, prepend_inner_header, FLAG_CE, FLAG_SP};
+use self::wire::{build_encrypted, build_established_header, prepend_inner_header, FLAG_CE, FLAG_KEY_EPOCH, FLAG_SP};
 use crate::{Config, ConfigError, Identity, IdentityError, NodeAddr, PeerIdentity};
 use rand::Rng;
 use std::collections::{HashMap, VecDeque};
@@ -1350,6 +1350,9 @@ impl Node {
         if ce_flag {
             flags |= FLAG_CE;
         }
+        if peer.current_k_bit() {
+            flags |= FLAG_KEY_EPOCH;
+        }
 
         let session = peer.noise_session_mut().ok_or_else(|| NodeError::SendFailed {
             node_addr: *node_addr,