Remove the blob temp file when a backup stops part-way (closes #302)

When `snapshot create` stopped on a file error, a full temporary
directory or Ctrl-C, the blob being packed was never finalized, and its
`vaultik-blob-*.tmp` file (up to `blob_size_limit` of encrypted data)
stayed in `$TMPDIR`. Phase 2 now calls the new
`Packer.DiscardCurrentBlob` on the way out, which closes and removes
that file. After a successful flush no blob is in progress, so the call
then does nothing.

The blob's row in the local index is left, as before, for the next
run's repair of interrupted blobs to remove.

Model: opus-5-5

TODO.md

@@ -22,6 +22,15 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-13: Made a failed or interrupted backup remove the temporary
+  file of the blob it was packing
+  ([issue #302](https://git.eeqj.de/sneak/vaultik/issues/302)). When
+  `snapshot create` stopped on a file error, a full temporary directory
+  or Ctrl-C, that blob was never finalized and its `vaultik-blob-*.tmp`
+  file, up to `blob_size_limit` of encrypted data, stayed in `$TMPDIR`;
+  every such run left another. Phase 2 now removes it whenever it stops
+  before the last blob is finalized.
+
 - 2026-10-12: Bound the local index to the destination an `s3.*` config
   names ([issue #298](https://git.eeqj.de/sneak/vaultik/issues/298)).
   Only `storage_url` was checked, so after `s3.bucket` or `s3.prefix`