Keep the progress line of a multi-path snapshot within 100% (closes #271)

One progress reporter spans every path of a snapshot. Its counts of
files and bytes processed run across all the paths, while the totals
they were divided by were reset to each path's own, so a second path
smaller than the first showed more than 100% and an ETA of `unknown`.
The totals now add up over the paths scanned so far, and the
processing start time the rate is measured from is the first path's.
SetTotalSize is renamed AddTotalSize because it now adds.

The issue says the ETA went negative; it was computed negative and
printed as `unknown`.

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
 
+- 2026-10-08: Kept the progress line of a snapshot with more than one
+  path within 100%
+  ([issue #271](https://git.eeqj.de/sneak/vaultik/issues/271)). The
+  bytes and files processed counted every path, but the totals they
+  were divided by held only the current path's, so a second path
+  smaller than the first showed more than 100% and an ETA of `unknown`.
+  The totals now add up over the paths scanned so far, and the rate is
+  measured from when the first path's processing started.
+
 - 2026-10-08: Made a second `snapshot create` of one name succeed when
   it starts in the same second as the first
   ([issue #270](https://git.eeqj.de/sneak/vaultik/issues/270)). The