MazeNET phase 2 step 6. Equips the repo layer with the CRUD the web
editor needs before deploy.
- TopologyNotEditable exception: raised when a pending-only method hits
a non-pending topology. The intent is "free-form edits stop at deploy;
the mutator (step 7) takes over for live topologies."
- _assert_pending helper checks status inside the session.
- update_lan / update_topology_decky accept enforce_pending=True for
pre-deploy callers (existing internal callers default to False so
behavior is unchanged).
- delete_lan: cascades edges; refuses if any decky has only one edge
(= this LAN is its home) to prevent orphans.
- delete_topology_decky: cascades edges.
- delete_topology_edge: bare-bones removal.
All four mutators accept expected_version for optimistic concurrency.
Existing tests continue to pass (no behavior change for persist/deploy).
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