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 is measured from the bytes processed since the current path's
scan phase ended, so a later path's scan phase does not lower it.
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

@@ -28,10 +28,9 @@ 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. 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.
+  The totals now add up over the paths scanned so far, and the rate
+  behind the ETA covers the current path's processing only, so a later
+  path's scan phase does not lower it.
 
 - 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 last scan phase ended, and
+	// Guarded by mu: when the current path's processing started, and
 	// BytesProcessed at that moment.
 	processStartTime  time.Time
 	processStartBytes int64
@@ -155,10 +155,9 @@ 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. Restarting the rate keeps a path's scan phase,
-// which processes nothing, out of the rate while that path is processed.
-// During a later path's scan phase the rate is still the previous
-// path's, and falls as that scan goes on.
+// so the total does too. The rate covers the current path's processing
+// only, so that the scan phase of a later path, which processes nothing,
+// does not lower it.
 func (pr *ProgressReporter) AddTotalSize(size int64) {
 	pr.stats.TotalSize.Add(size)
 
@@ -169,8 +168,9 @@ func (pr *ProgressReporter) AddTotalSize(size int64) {
 	pr.stats.processStartBytes = pr.stats.BytesProcessed.Load()
 }
 
-// processRate returns the bytes processed per second since the last scan
-// phase ended, or 0 before the first path's scan phase is done.
+// processRate returns the bytes processed per second since the current
+// path's processing started, or 0 before the first path's scan phase is
+// done.
 func (s *ProgressStats) processRate() float64 {
 	s.mu.RLock()
 	defer s.mu.RUnlock()