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

report.go

@@ -76,7 +76,7 @@ func runReport(ctx context.Context, stdout io.Writer) error {
 
 	prog = newProgress(progressParams{
 		label: reportPhase("read"), unit: "records", total: -1,
-		waitingFor: countingRecords,
+		waitingFor: countingRecords, doing: readingDatabase,
 	})
 	report, err := readReport(ctx, db, dbPath, stdout, prog)