v0.6.0: primary-link failover mode, peer-signaled primary, peer MTU display

- scheduler.primary_link_mode: single-link failover with hard-Down
  immediate switching and margin/streak hysteresis to prevent flapping
  between close-scoring links.
- scheduler.follow_peer_primary: one host signals its chosen primary
  link (by name) to its peer over the wire, so the peer can follow
  instead of running its own independent primary-selection config.
- mlvpn-tui shows both sides' live tunnel MTU in the Session panel.
- VersionInfoPayload's decode is now tolerant of shorter payloads from
  pre-0.6.0 peers, avoiding a silent blind spot during rolling upgrades.