report: show its phases as N of M, with progress, elapsed time and ETA (closes #114)

report now runs in two phases, open and read, under the progress
display scan and trees use, until the summary line. open covers a wait
on a busy database. read counts the records, then reads them in the
same read-only transaction in signature order, the files_signature
index read backwards, so SQLite sorts nothing and the read ends at
exactly its count. The duplicate groups are formed in Go from runs of
equal signatures; only those of the size being read are held, and they
are written, ordered by first path, before the next size. The rows go
through the display's stdout writer. stdout and the summary line are
unchanged.

Judgement call: writing is part of read, not a phase of its own, since each size's rows are written while the read goes on.

Model: opus-5-5

scan.go

@@ -446,7 +446,7 @@ func (s *scanState) loadIndex(ctx context.Context, roots []string) error {
 	// first gives the display its total.
 	prog := newProgress(progressParams{
 		label: scanPhase("load"), unit: scanUnit, total: -1,
-		waitingFor: "counting the records",
+		waitingFor: countingRecords,
 	})
 	defer prog.finish()