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.go

@@ -72,7 +72,7 @@ func runTrees(ctx context.Context, stdout io.Writer) error {
 
 	prog = newProgress(progressParams{
 		label: treesPhase("read"), unit: "records", total: -1,
-		waitingFor: countingRecords,
+		waitingFor: countingRecords, doing: readingDatabase,
 	})
 	records, super, allDirs, err := readTree(ctx, db, prog)
 
@@ -84,7 +84,7 @@ func runTrees(ctx context.Context, stdout io.Writer) error {
 
 	prog = newProgress(progressParams{
 		label: treesPhase("group"), unit: "directories",
-		total: int64(len(allDirs)),
+		total: int64(len(allDirs)), doing: "grouping directories",
 	})
 	dupes := collectTreeGroups(allDirs, super, prog)
 
@@ -92,6 +92,7 @@ func runTrees(ctx context.Context, stdout io.Writer) error {
 
 	prog = newProgress(progressParams{
 		label: treesPhase("write"), unit: "groups", total: int64(len(dupes)),
+		doing: "writing the report",
 	})
 	dupeTrees, reclaimable, err := writeTreeGroups(stdout, dupes, prog)