Decomposes the M5 mesh-hardening follow-on into 3 independent sub-specs; this
is sub-project 1. Three-layer SSH-on-wt0 (sshd ListenAddress=mesh + nftables
iifname wt0 + retire the Hetzner WAN :22), ip_nonlocal_bind to beat the
post-boot wt0 bind race (fail-closed), live wt0 fact for the listen addr,
staged cutover with the firewall auto-rollback as the safety gate.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
