trees: show its phases as N of M, with progress, elapsed time and ETA (closes #113)

trees now runs in four phases, open, read, group and write, each under
the progress display scan uses, until the summary line. open covers a
wait on a busy database. read counts the records and reads them in one
read-only transaction, so the two agree during a scan; group counts the
directories, and write the groups. The rows reach stdout through a
writer that clears the display and holds its lock while writing, so no
row shares a line with the display on a shared terminal. Each rate
names what its phase counts.

Judgement call: opening the database is a phase of its own, so there are four phases, not the three the issue sketches.

Model: opus-5-5

scan.go

@@ -84,6 +84,10 @@ type fileMeta struct {
 	hashed bool
 }
 
+// scanUnit is what every scan phase counts, as its progress display
+// names it.
+const scanUnit = "files"
+
 // scanPhase names one phase of a scan with its place among all of them,
 // as "[2/5] walk", for the phase's progress display. The list here is
 // the one place the phases are counted.
@@ -441,7 +445,7 @@ func (s *scanState) loadIndex(ctx context.Context, roots []string) error {
 	// display the scan looks hung before the walk begins. Counting them
 	// first gives the display its total.
 	prog := newProgress(progressParams{
-		label: scanPhase("load"), total: -1,
+		label: scanPhase("load"), unit: scanUnit, total: -1,
 		waitingFor: "counting the records",
 	})
 	defer prog.finish()
@@ -492,7 +496,8 @@ func (s *scanState) walkPhase(
 	}
 
 	prog := newProgress(progressParams{
-		label: scanPhase("walk"), total: estimate, estimate: true,
+		label: scanPhase("walk"), unit: scanUnit, total: estimate,
+		estimate: true,
 	})
 
 	var (
@@ -683,7 +688,7 @@ func (s *scanState) readRuns(ctx context.Context, workers int,
 	defer pool.stop()
 
 	prog := newProgress(progressParams{
-		label: scanPhase(label), total: int64(len(runs)),
+		label: scanPhase(label), unit: scanUnit, total: int64(len(runs)),
 	})
 	defer prog.finish()
 
@@ -772,7 +777,7 @@ func (s *scanState) updatePhase(ctx context.Context) error {
 
 	total := len(s.batch) + len(s.sentinels) + len(deletes)
 	prog := newProgress(progressParams{
-		label: scanPhase("update"), total: int64(total),
+		label: scanPhase("update"), unit: scanUnit, total: int64(total),
 	})
 
 	defer prog.finish()
@@ -860,7 +865,7 @@ func (s *scanState) contentCandidates(
 	// record. The checks start there, against a count of the records
 	// the search returns, and so do their rate and ETA.
 	prog := newProgress(progressParams{
-		label: scanPhase("content"), total: -1,
+		label: scanPhase("content"), unit: scanUnit, total: -1,
 		waitingFor: "searching the database",
 	})
 	defer prog.finish()