Run 1 was a test artifact (probe used DHCP; uplink /24 has no <dhcp> by
design -- DOCFIX queued for the gate wording). Run 2 static: gateway
ping 3/3 across the nested bridge = the D-125 property PASSES; 1.1.1.1
fails for ICMP and TCP alike. Controls pass and net-dumpxml shows the
working/failing NATs are substantively identical, so the suspect is
vcloud's live virbr4 rules, not the design -- double-NAT fallback
deliberately NOT invoked. Probe guest + temp addresses removed.
CURRENT-STATE updated in this commit (GA-R1/C1).

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