dc-node-v6-carve.py: honour MAAS_PROFILE; refuse instead of tracebacking ...

The script was env-blind -- default 'admin', --profile only, no environment
read anywhere. Every bash tool here honours MAAS_PROFILE, so
`export MAAS_PROFILE=vr1-dc0-region` left THIS tool silently targeting the
Office1 region. During the per-DC region migration that is the
unrecoverable direction: against the wrong region a carve is an idempotent
no-op that prints PASS while the region you meant stays untouched.

It now reads MAAS_PROFILE, keeps --profile as an override, and prints which
region it targets and where that came from -- a capture that does not name
the profile cannot be audited for wrong-region work.

An absent maas CLI raised FileNotFoundError and exited 1 with a traceback,
indistinguishable from a real failure and contrary to this script's own
REFUSE contract. It now refuses with exit 2.

Harness 14/14 (was 9); both fixes proven able to fail.

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

dc-node-v6-carve harness 9/9; manifest re-recorded (84) ...

Locks the properties that matter: the octet mirror is TEXTUAL (.121 -> ::121,
never a hex conversion); the D-100 raw provider NIC is skipped for having no v4
link while br-ex is included for having one; an interface already carrying a
DIFFERENT v6 refuses rather than adding a second; a vlan with no v6 subnet is a
named error rather than a silent skip; a tag matching no machines REFUSES rather
than reporting a clean zero; and apply is dry by default.

T5 drives a fully-carved fixture to green -- every live run currently fails
because nothing is carved, and a gate only ever observed failing is as
untrustworthy as one only ever observed passing.

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