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

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)