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

The rate was the phase's average, so a burst after a long wait read as
over 1,000 files a second. It now covers the last 5 seconds, or the
time since the previous line off a terminal; the ETA still comes from
the average. Once no item has completed for 10 seconds, the rate gives
way to "stalled" and where the phase waits: disk, db, out (stdout) or
cpu, or two of them, as "stalled disk/db". A line too wide for the
terminal loses its bar first, then its rate or those words, so the
phase, progress, elapsed time and ETA stay whole. The content phase's
last partial batch is now written under a display of its own.

Deviation: the stall words hold no time and name where the phase waits, not what it does.
Judgement call: trees' group phase, which works in memory, says "cpu".

Model: opus-5-5

report.go

@@ -74,9 +74,11 @@ func runReport(ctx context.Context, stdout io.Writer) error {
 
 	defer func() { _ = db.Close() }()
 
+	// The read phase writes the report to stdout while it reads the
+	// records, so it may wait on either.
 	prog = newProgress(progressParams{
 		label: reportPhase("read"), unit: "records", total: -1,
-		waitingFor: countingRecords,
+		waitingFor: countingRecords, stalledOn: "db/out",
 	})
 	report, err := readReport(ctx, db, dbPath, stdout, prog)