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, from the line naming the database to
the summary line. open covers a wait on a busy database. read counts
the records first, as scan's load does, then reads them against that
count; group counts the directories, and write the groups. The display
gains a unit so that each rate names what its phase counts; every scan
phase passes files and prints as before. The tests that pinned trees'
stderr now allow the phase lines between the database line and the
summary.

Judgement call: opening the database is a phase of its own, so there are four phases, not the three the issue sketches.
Judgement call: the count and the read are separate queries, as in scan's load, so during a scan the read can end slightly past or short of its total.

Model: opus-5-5

scan_test.go

@@ -1550,7 +1550,7 @@ func assertSmokeTreeGroups(t *testing.T, dir string, db *sql.DB) {
 
 	super, dirs := dbTree(t, db)
 
-	tg := collectTreeGroups(dirs, super)
+	tg := collectTreeGroups(dirs, super, nil)
 	if len(tg) != 1 {
 		t.Fatalf("len(tree groups) = %d, want 1", len(tg))
 	}
@@ -2092,7 +2092,7 @@ func TestTreesUnhashedNeverEqual(t *testing.T) {
 	for name, unknown := range cases {
 		super, dirs := treeOf(t, append(slices.Clone(shared), unknown...))
 
-		if tg := collectTreeGroups(dirs, super); len(tg) != 0 {
+		if tg := collectTreeGroups(dirs, super, nil); len(tg) != 0 {
 			t.Errorf("%s: tree groups = %d, want 0 (the files may differ)",
 				name, len(tg))
 		}