v0.7.3-beta3: fixes a real production crash found during a fec_group_size sweep fec_group_size=16 crash-looped mlvpnd on both home-bgp and cloud-bgp (TUN device creation EINVAL) because the dual-stack (IPv6) MTU floor was wrong (576 instead of the real, mandatory 1280 per RFC 8200). effective_tunnel_mtu now floors correctly for any tunnel with address6 configured, and warns when FEC's own reservation can't be fully honored at that floor instead of silently under-reserving.