DOCFIX-152: NetBox multi-DC/dual-stack import pipeline (gap #3 mechanism) ...

Adds netbox/dc-dc-prefixes-import.py, extending the v1 single-site IPv4-only
NetBox import to VR1's two-DC dual-stack model (D-101). DC1's v4 planes are
hardcoded (D-101: inherited from DC0 unchanged, this is decision text, not
an inference); DC2's v4 supernet, the org ULA /48, and the per-DC GUA carve
are all required env vars with no defaults, failing loud rather than
inventing any of them. 37/37 tests against a fake in-memory NetBox client
(no live NetBox reachable this session). Closes the tooling half of gap #3;
the real literals still need operator/NetBox assignment.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QrcJx8TUar7pYAvpGJw57A