Write rclone uploads under a temporary name, re-upload short blobs (closes #266)

The rclone backend wrote each object straight to its key, so killing an
upload to a local or sftp remote left a truncated object there. The next
backup found the key, skipped the upload and recorded a snapshot that
could not be restored.

On a remote where rclone says a file can be seen while it is still being
written, an object is now written under a name ending in `.partial` and
moved onto its key with the remote's server-side move, as rclone's own
copy does; listings skip such names. A backup also uploads a blob again
when the stored object's size differs from the blob's.

Judgement call: the temporary name is used only where rclone sets its
PartialUploads feature; other remotes already show an object only once
complete.

Model: opus-5-5

TODO.md

@@ -22,6 +22,18 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-07: Stopped a backup from trusting a blob object left short by
+  a killed upload
+  ([issue #266](https://git.eeqj.de/sneak/vaultik/issues/266)). The
+  rclone backend wrote each object straight to its key, so killing an
+  upload to a local or sftp remote left a truncated object there; the
+  next backup found the key with `Stat`, skipped the upload and recorded
+  a snapshot that could not be restored. On a remote where rclone says a
+  file can be seen while it is being written, an object is now written
+  under a temporary name ending in `.partial` and moved into place, and
+  listings skip such names. A backup also uploads a blob again when the
+  stored object's size differs from the blob's.
+
 - 2026-10-07: Corrected documentation, help text and comments that were
   false about the code
   ([issue #233](https://git.eeqj.de/sneak/vaultik/issues/233)). A blob