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. The rate behind
the ETA restarts when each path's scan phase ends, so that phase does
not lower the rate while the path is processed; until then the rate is
the previous 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

@@ -28,9 +28,10 @@ the tag exists and is exercised; what is left is merging `next` to
   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
+  The totals now add up over the paths scanned so far. The rate behind
-  behind the ETA covers the current path's processing only, so a later
+  the ETA restarts when each path's scan phase ends, so that phase does
-  path's scan phase does not lower it.
+  not lower the rate while the path is processed; until then the rate
+  is the previous path's.
 
 - 2026-10-08: Made a second `snapshot create` of one name succeed when
   it starts in the same second as the first

internal/snapshot/progress.go

@@ -73,7 +73,7 @@ type ProgressStats struct {
 	mu             sync.RWMutex
 	lastDetailTime time.Time
 
-	// Guarded by mu: when the current path's processing started, and
+	// Guarded by mu: when the last scan phase ended, and
 	// BytesProcessed at that moment.
 	processStartTime  time.Time
 	processStartBytes int64
@@ -155,9 +155,10 @@ func (pr *ProgressReporter) GetStats() *ProgressStats {
 // AddTotalSize adds the size one path of the snapshot has to process to
 // the total, once that path's scan phase is done, and starts measuring
 // the processing rate again. The processed counts run across every path,
-// so the total does too. The rate covers the current path's processing
+// so the total does too. Restarting the rate keeps a path's scan phase,
-// only, so that the scan phase of a later path, which processes nothing,
+// which processes nothing, out of the rate while that path is processed.
-// does not lower it.
+// During a later path's scan phase the rate is still the previous
+// path's, and falls as that scan goes on.
 func (pr *ProgressReporter) AddTotalSize(size int64) {
 	pr.stats.TotalSize.Add(size)
 
@@ -168,9 +169,8 @@ func (pr *ProgressReporter) AddTotalSize(size int64) {
 	pr.stats.processStartBytes = pr.stats.BytesProcessed.Load()
 }
 
-// processRate returns the bytes processed per second since the current
+// processRate returns the bytes processed per second since the last scan
-// path's processing started, or 0 before the first path's scan phase is
+// phase ended, or 0 before the first path's scan phase is done.
-// done.
 func (s *ProgressStats) processRate() float64 {
 	s.mu.RLock()
 	defer s.mu.RUnlock()