finalizeSnapshotMetadata marked the snapshot complete and then exported its
metadata. A crash after completion but before/during the export left the
local index showing the snapshot as complete while the destination had no
manifest or database, and PruneDatabase (which drops only NULL completed_at
rows) kept it: a silently unrestorable snapshot.
Reorder so completion is recorded last. CompleteSnapshot is split into
PopulateSnapshotBlobs (before the export, which reads snapshot_blobs) and
MarkSnapshotComplete (after it). An interrupted export now leaves the
snapshot incomplete, so the next run's PruneDatabase drops it and re-backs-up
the data. The reverse tiny window leaves a fully restorable snapshot at the
destination that the index reports honestly as remote-only.
Update REPOSTRUCTURE.md guarantee 4 and the ARCHITECTURE.md flow to the new
order. Add a fault-injection test driving the full create path.
Model: opus-4-8
sneak