Mark a snapshot complete only after its metadata export succeeds (closes #177)

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

docs/REPOSTRUCTURE.md

@@ -153,7 +153,7 @@ These are known, deliberate properties of the format and the tooling, recorded s
 1. **Blobs are immutable** - Once written, a blob is never modified
 2. **Blobs are written before metadata** - A snapshot's metadata is only written after all its blobs are successfully uploaded
 3. **Metadata is written atomically** - Both db.zst.age and manifest.json.zst are written as complete files
-4. **A snapshot is marked complete in the local DB before its metadata is uploaded, not after** - `CompleteSnapshot` runs first, then `ExportSnapshotMetadata` (see the backup data flow in [ARCHITECTURE.md](../ARCHITECTURE.md)). A crash between the two leaves a completed-looking row in the local index with no matching metadata on the destination store. `vaultik prune` reconciles this away: it drops any local snapshot whose remote metadata is missing.
+4. **A snapshot is marked complete in the local DB only after its metadata is uploaded** - `finalizeSnapshotMetadata` runs `ExportSnapshotMetadata` first and records completion (`MarkSnapshotComplete`) only once the export succeeds (see the backup data flow in [ARCHITECTURE.md](../ARCHITECTURE.md)). A crash during the export therefore leaves the snapshot incomplete, so the next backup's `PruneDatabase` drops it and re-backs-up its data, rather than leaving a completed-looking row in the local index with no matching metadata on the destination store. (A crash in the brief moment after the export succeeds but before completion is recorded leaves a fully-restorable snapshot on the destination that the local index drops as incomplete on the next run; `snapshot list` then reports it honestly as remote-only, which is the safe direction: the destination copy stays restorable.)
 
 ## Pruning Safety