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