DOCFIX-157/158/159: Stage 4/6/7 runbooks -- MAAS deploy, DR drill, Designate/COS/Magnum ...

Completes all seven Stage 1-7 DC-DC runbooks (tooling gap register item #9
CLOSED):
- Stage 4 (MAAS enlist/commission/deploy, per DC): adapts phase-00's
  validated single-rack sequence per DC. Mandates the $DC selector
  convention (DOCFIX-151) and honestly documents that lib_hosts_select_dc
  will keep failing until this stage's own enrollment populates real host
  data. Names 7 real multi-rack gaps rather than working around them.
- Stage 6 (DR wiring and failover drill): implements Section 8's
  failover/failback skeleton, one-way-then-two-way per D-108. Found three
  real bundle.yaml gaps (missing cinder-backup/ceph-rbd-mirror charms,
  wrong-plane rbd-mirror binding) and makes fixing them the runbook's own
  first gated step. Includes a negative-control drill before the real one.
- Stage 7 (Designate, COS, Magnum/CAPI): quotes the bundle's actual
  current "NO designate" text before describing the D-106 reversal;
  identifies the reversal also affects the bundle's separate
  os-public-hostname posture; leaves COS's mechanism as an operator
  decision rather than inventing one.

Four new tooling-gap-register items (#12-15) added for cross-cutting
findings this runbook-authoring pass surfaced. None of the seven runbooks
have been executed against real infrastructure yet -- prep-only session.

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