beta4: primary-aware, peer-negotiated tunnel MTU Includes the full primary-aware MTU feature (effective_tunnel_mtu's primary_idx parameter, negotiate_tunnel_mtu peer negotiation, faster 3s mtu_monitor_loop poll) plus the final-review fix for the monotonic MTU ratchet bug (local_capability_mtu advertised to the peer instead of the already-negotiated value) and the floor-clamping fix. docs/superpowers/specs/2026-09-16-primary-aware-peer-mtu-design.md docs/superpowers/plans/2026-09-16-primary-aware-peer-mtu.md