gateway-lan pinned 172.20.1.0/24 and fd02::/64, so two concurrent local CI
runs collided on "Pool overlaps" at network creation. The IPv4 subnet has no
consumer -- the whole gateway LAN path is IPv6 -- so drop it and let docker
auto-assign. The IPv6 side cannot float, because the LAN clients' resolv.conf
pins the gateway's address as their nameserver and that must be a literal
known before they start, so claim a free /64 per run and thread the prefix
through the compose address pins, a generated resolv.conf, and the test via a
single exported variable.
The claim and the external network ship as a harness-only compose overlay
applied by run_gateway; the base compose keeps a normal gateway-lan network,
so the GitHub matrix and any standalone bring-up are unaffected. With no claim
every address renders exactly as before.
