MazeNET phase 2 step 4. Readies the repo layer for concurrent editors
(web canvas + CLI + mutator) without lost-write races.
- Topology.version: monotonically bumped on supervised child-row writes.
- VersionConflict exception carries {current, expected} for the UI.
- _check_and_bump_version helper reads Topology in the same session,
compares against expected_version, raises on mismatch, bumps on match.
Commit happens in the caller's existing transaction so check+bump+write
are atomic per mutation.
- add_lan / update_lan / add_topology_decky / update_topology_decky /
add_topology_edge accept expected_version=None by default, preserving
every existing caller's behavior.
When expected_version is None, no check runs and version stays put —
internal callers (persist) that don't care about concurrency keep
working unchanged.
