Start the progress reporter once per snapshot, not once per path (closes #253)

A backup without --cron of a snapshot with two or more paths panicked
with "close of closed channel". Scan runs once per path, and it started
the progress reporter and deferred its Stop each time; Stop closes the
reporter's signal channel, so the second path's Stop panicked.
scanAllDirectories now starts the reporter before the first path and
stops it after the last, and Scan no longer starts or stops it. A new
test backs up a two-path snapshot with the reporter on and restores
both paths.

Model: opus-5-5

TODO.md

@@ -22,6 +22,14 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-06: Made a backup without `--cron` of a snapshot with two or
+  more `paths` complete instead of panicking with `close of closed
+  channel` ([issue #253](https://git.eeqj.de/sneak/vaultik/issues/253)).
+  `Scan` runs once per path and started and stopped the progress
+  reporter each time, and a second stop panics. The reporter is now
+  started and stopped once per snapshot, around the scans of all its
+  paths.
+
 - 2026-10-06: Made a restore path argument select only that path and
   what is beneath it
   ([issue #223](https://git.eeqj.de/sneak/vaultik/issues/223)). The