Show the progress rate over the last 5 seconds, and what a phase is doing after 10 seconds with no item done (closes #125)

A display's rate was the phase's average since it started, so after a
long wait a burst of files read as a steady rate of over 1,000 files a
second. It now covers the items completed over the last 5 seconds, or
since the line before off a terminal, and falls to 0 while nothing
completes; the ETA still comes from the average. Once no item has
completed for 10 seconds, the line says so and what the phase is doing,
a phrase each phase gives where it is defined. A narrow terminal leaves
that out after the rate, keeping the phase, progress, elapsed time and
ETA whole. The tests set the display's clock by hand.

Judgement call: the line reads "no files finished", in the phase's unit.
Judgement call: trees' group phase says "grouping directories".

Model: opus-5-5

scan.go

@@ -88,6 +88,15 @@ type fileMeta struct {
 // names it.
 const scanUnit = "files"
 
+// readingFiles is what the display of a phase that reads files says it
+// is doing once no file has been read for stallTime.
+const readingFiles = "reading files"
+
+// searchingDatabase is what the display of the content phase's checks
+// says it waits for until the search returns its first record, and what
+// it says it is doing once no record has been checked for stallTime.
+const searchingDatabase = "searching the database"
+
 // scanPhase names one phase of a scan with its place among all of them,
 // as "[2/5] walk", for the phase's progress display. The list here is
 // the one place the phases are counted.
@@ -446,7 +455,7 @@ func (s *scanState) loadIndex(ctx context.Context, roots []string) error {
 	// first gives the display its total.
 	prog := newProgress(progressParams{
 		label: scanPhase("load"), unit: scanUnit, total: -1,
-		waitingFor: countingRecords,
+		waitingFor: countingRecords, doing: readingDatabase,
 	})
 	defer prog.finish()
 
@@ -497,7 +506,7 @@ func (s *scanState) walkPhase(
 
 	prog := newProgress(progressParams{
 		label: scanPhase("walk"), unit: scanUnit, total: estimate,
-		estimate: true,
+		estimate: true, doing: "listing directories",
 	})
 
 	var (
@@ -689,6 +698,7 @@ func (s *scanState) readRuns(ctx context.Context, workers int,
 
 	prog := newProgress(progressParams{
 		label: scanPhase(label), unit: scanUnit, total: int64(len(runs)),
+		doing: readingFiles,
 	})
 	defer prog.finish()
 
@@ -778,6 +788,7 @@ func (s *scanState) updatePhase(ctx context.Context) error {
 	total := len(s.batch) + len(s.sentinels) + len(deletes)
 	prog := newProgress(progressParams{
 		label: scanPhase("update"), unit: scanUnit, total: int64(total),
+		doing: "writing to the database",
 	})
 
 	defer prog.finish()
@@ -866,7 +877,7 @@ func (s *scanState) contentCandidates(
 	// the search returns, and so do their rate and ETA.
 	prog := newProgress(progressParams{
 		label: scanPhase("content"), unit: scanUnit, total: -1,
-		waitingFor: "searching the database",
+		waitingFor: searchingDatabase, doing: searchingDatabase,
 	})
 	defer prog.finish()