Keep the record count and total size in a totals table (closes #146)

info counted the records and added up their sizes with a query that
reads every record, which takes minutes on a large database. The
schema now has a totals table of one row, kept current by three
triggers on files, in the transaction of each insert, delete and size
update. info, scan's load phase and the read phases of report and
trees read that row instead of counting over files. scan writes files
with the same statements as before. Each of those writes now fires a
trigger and is slower, so scan keeps SQLite's temporary data in
memory, which wins back part of that time.

Model: opus-5-5

info.go

@@ -26,11 +26,11 @@ func infoPhase(name string) string {
 // it. Any database problem, including a missing database, is fatal.
 //
 // The work runs in two phases, each under a progress display of its
-// own on stderr: open (open the database) and count (count the records,
-// with one query). Each display ends before the next phase starts, and
-// before an error is returned to be printed. The details are written
-// once the last display has ended, so no display is drawn while they
-// are written.
+// own on stderr: open (open the database) and count (read the number of
+// records and their total size from the totals table). Each display
+// ends before the next phase starts, and before an error is returned
+// to be printed. The details are written once the last display has
+// ended, so no display is drawn while they are written.
 func runInfo(ctx context.Context, stdout io.Writer) error {
 	dbPath, source, err := databasePath()
 	if err != nil {
@@ -56,7 +56,7 @@ func runInfo(ctx context.Context, stdout io.Writer) error {
 
 	defer func() { _ = db.Close() }()
 
-	// One query counts everything, so the phase never has a total.
+	// One row holds both counts, so the phase never has a total.
 	prog = newProgress(progressParams{
 		label: infoPhase("count"), total: -1,
 		waitingFor: countingRecords,