D-124 addressing pinned: transit supernet 172.31.0.0/24 (operator-ratified); importer CONTAINER + harness ...

Operator ruled the three dc-rack-mgmt-import.py scoping flags + ratified the
values (2026-07-16): seed a dedicated `transit` role; CARVE a dedicated transit
supernet (not Cloud) = 172.31.0.0/24; scope vr1-dc0 ("mirror the edge"); transit
/30 172.31.0.0/30; rack IP 10.12.8.2.

- netbox/dc-rack-mgmt-import.py: CONTAINER "10.12.0.0/16" -> "172.31.0.0/24"
  (+ header note, RULED scope flags, ratified usage example). Still takes
  --transit-cidr/--rack-ip as required inputs; still fail-loud on missing/
  collision container (the backstop for a block picked against a stale draft).
- tests/dc-rack-mgmt-import/{run-tests.sh,test_logic.py}: CONTAINER assertion +
  every transit fixture rebased Cloud->172.31.0.x (shapes/host-bits preserved so
  rejections still die for the right reason). 96/96 PASS.
- docs: D-124 AMENDMENT (carve + ratified addressing) + changelog-20260716-
  d124-addressing-pin.md (the exact on-host --commit sequence) + session-ledger.

NOT DONE (cannot from here): the NetBox --commit. office1-netbox (10.10.1.10) is
UNREACHABLE from the vcloud jumphost (ping/TCP:8000/HTTP all fail, no SSH/tunnel)
and the apex token is a secret on that host -- the write is an on-host operator
step. NO apex mutation in this commit.

Gauntlet ALL GREEN (63); repo-lint 0 fail.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ck6xh3jWQi5b3Su8Dx1LEH