Leave remote info orphan figures unknown when a manifest is unreadable (closes #228)

When a manifest could not be read, remote info skipped it, counted that
snapshot's blobs as orphaned and advised running prune. The orphan
figures are now unknown in that case, with no prune advice; --json gives
them as null and lists the unreadable remote keys in
unreadable_manifests.

Names under metadata/ were used unchecked and printed raw. A name that
is not a remote key is now skipped with a warning. Its manifest is then
not read either, so a skipped name also leaves the orphan figures
unknown; --json counts such names in skipped_metadata_names without
printing them.

The closing log line carries the unreadable manifest count in place of
the orphan count.

Model: opus-5-5

TODO.md

@@ -22,6 +22,20 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-07: Made `remote info` stop reporting a snapshot's blobs as
+  orphaned when its manifest cannot be read, and stop printing raw
+  names from under `metadata/`
+  ([issue #228](https://git.eeqj.de/sneak/vaultik/issues/228)). A
+  manifest it failed to read was skipped, so that snapshot's blobs were
+  counted as orphaned and the report advised running `vaultik prune`.
+  The orphan figures are now unknown in that case, with no prune
+  advice, and `--json` gives them as `null` with the unreadable remote
+  keys in `unreadable_manifests`. A name under `metadata/` that is not
+  64 lowercase hex characters is now skipped with a warning instead of
+  being printed, control characters included. Its manifest is then not
+  read either, so a skipped name also leaves the orphan figures unknown,
+  and `--json` counts such names in `skipped_metadata_names`.
+
 - 2026-10-07: Made a backup notice a file rewritten with its size
   unchanged and a new mtime in the same second as the one in the index
   ([issue #226](https://git.eeqj.de/sneak/vaultik/issues/226)). The