The rack has no default route (transit-only netplan; WAN bridge IP-less),
so 172.30.2.51-sourced traffic to 1.1.1.1 fails in the rack's own routing
table before the NAT is consulted. Its ICMP/TCP failures say nothing about
virbr4; the only clean signal is the guest's ICMP failure, and a guest TCP
datapoint is still missing. Measurement request widened accordingly: FULL
nft AND iptables-save dumps (backend unmeasured), diffing virbr4 vs
virbr11 across BOTH rule families -- POSTROUTING masquerade (subnet-keyed)
and FORWARD accept (interface-keyed, which a subnet grep would miss).
Fix-forward pre-staged: net-destroy/net-start reinstalls libvirt's rules.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KiUu1oqt76tWvV4vEC3NAr