Show the progress rate over the last 5 seconds, and what a phase is doing 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 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_test.go

@@ -156,7 +156,7 @@ func TestReportStdoutFailsWhileReading(t *testing.T) {
 	}
 
 	// Counts the records read, and draws nothing.
-	prog := &progress{}
+	prog := &progress{now: time.Now}
 
 	_, err = readReport(t.Context(), db, "", failingWriter{}, prog)
 	if !errors.Is(err, errWriteFailed) ||
@@ -250,7 +250,7 @@ func TestReadReportOnTerminal(t *testing.T) {
 	p := startProgress(progressParams{
 		label: reportPhase("read"), unit: "records", total: -1,
 		waitingFor: countingRecords,
-	}, terminalWidth, time.Hour)
+	}, terminalWidth, time.Hour, time.Now)
 
 	_, err = readReport(t.Context(), db, "", os.Stderr, p)