DOCFIX-160: adversarial review pass finds and fixes 3 real issues ...

Ran a fresh-eyes review subagent across tonight's full deliverable set
(lib-net.sh/lib-hosts.sh selectors, the NetBox DC-DC pipeline, all seven
Stage 1-7 runbooks). Found and fixed:

- A real Python control-flow bug in tests/dc-dc-prefixes-import/
  test_logic.py: a `return` inside `try` made the `else` clause
  unreachable, silently leaving 3 happy-path assertions uncounted.
  37/37 -> 40/40 after the fix (independently reproduced before trusting
  the finding).
- A self-contradictory pair of comments in dc-dc-prefixes-import.py about
  /19-vs-/22 subnet arithmetic (no functional bug, wrong prose).
- Stale self-referential text in the Stage 4 runbook, written before
  Stage 3 existed and never reconciled once it did.

Corrected the 40/40 count in the workflow tracker and session ledger.

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

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