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

trees_test.go

@@ -279,7 +279,7 @@ func TestWriteTreeGroupsOnTerminal(t *testing.T) {
 	// No timed draws: the display is drawn as it starts and as it ends.
 	p := startProgress(progressParams{
 		label: treesPhase("write"), unit: "groups", total: int64(len(dupes)),
-	}, terminalWidth, time.Hour)
+	}, terminalWidth, time.Hour, time.Now)
 
 	_, _, err := writeTreeGroups(os.Stderr, dupes, p)