dc-node-carve.sh: the v4 node interface carve, mutation-proven 40/40 ...

The last no-tool gap between the dc0 region migration and the Juju deploy:
60 NIC re-homes, 9 OVS br-ex bridges, 54 statics. The Stage-4 equivalent was
done ad-hoc and logged outside the repo, so there was nothing to re-run when
D-132 q1 forced a re-enrolment.

Proved the four-call MAAS cycle on ONE live interface (canary 6q4syf enp3s0)
before writing the tool -- nothing had exercised `interface update vlan=` on a
link_up-only interface on an auto fabric, which is the state all 60 NICs are in.
Measured in passing: link-subnet REPLACES a link_up link, so only the br-ex
member needs an explicit unlink.

No default profile; maas-profile-assert.sh before any read or write; every id
resolved live by stable key (boot MAC / CIDR / interface name); every mutation
read back and compared. Role asymmetry derived from the host token, and the
controller's deliberately-uncarved NICs are asserted ABSENT.

Mutation pass found the br-ex static compare could not fail -- Pattern B was
being caught by the type/parent assertions instead. Added the brexip fixture
and re-proved the kill. Also found that neutering an assertion's MESSAGE only
proves it exists, not that its predicate works.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HvCyrwvYTTcDYnRErfMsNf