Implement persistent SQLite scan database

scan now synchronizes a database that survives between runs
(SFDUPES_DATABASE, default /var/lib/sfdupes/db.sqlite) instead of
emitting a stream: operands are resolved to absolute paths, unchanged
files (same size, mtime not newer than recorded) are never re-read, new
and changed files are hashed, and records under the scanned operands
that were not verified this run are deleted; records outside the
operands are untouched. All changes commit in a single transaction, and
WAL journaling with a busy timeout keeps a report run during a cron
scan safe.

report and trees read the database (no positional arguments); the
NUL-terminated stream format, its parser, and the malformed-record
handling are gone. The driver is modernc.org/sqlite (pure Go), so
builds keep cgo disabled.

main.go

@@ -2,13 +2,15 @@
 // very large filesystems without reading full file contents. Files are
 // considered duplicates when they have identical size, identical SHA-256
 // of their first 1024 bytes, and identical SHA-256 of their last 1024
-// bytes.
+// bytes. scan maintains a persistent SQLite database of file signatures
+// (SFDUPES_DATABASE, default /var/lib/sfdupes/db.sqlite) that the
+// reporting subcommands read.
 //
 // Usage:
 //
-//	sfdupes scan [--workers N] [-x] PATH... > files.dat
-//	sfdupes report [files.dat|-] > dupes.tsv
-//	sfdupes trees [files.dat|-] > dupetrees.tsv
+//	sfdupes scan [--workers N] [-x] PATH...
+//	sfdupes report > dupes.tsv
+//	sfdupes trees > dupetrees.tsv
 //
 // See README.md for the complete specification.
 package main
@@ -60,7 +62,7 @@ func main() {
 
 	scanCmd := &cobra.Command{
 		Use:   "scan [--workers N] [-x] PATH...",
-		Short: "Walk trees and emit one record per regular file on stdout",
+		Short: "Walk trees and synchronize the scan database",
 		Args:  cobra.MinimumNArgs(1),
 		Run: func(_ *cobra.Command, args []string) {
 			runScan(args, scanWorkers, scanOneFS)
@@ -72,20 +74,20 @@ func main() {
 		"do not cross filesystem boundaries")
 
 	reportCmd := &cobra.Command{
-		Use:   "report [files.dat|-]",
-		Short: "Read a scan stream and print the file-level duplicates report",
-		Args:  cobra.MaximumNArgs(1),
-		Run: func(_ *cobra.Command, args []string) {
-			runReport(args)
+		Use:   "report",
+		Short: "Read the scan database and print the file-level duplicates report",
+		Args:  cobra.NoArgs,
+		Run: func(_ *cobra.Command, _ []string) {
+			runReport()
 		},
 	}
 
 	treesCmd := &cobra.Command{
-		Use:   "trees [files.dat|-]",
-		Short: "Read a scan stream and print the duplicate-tree report",
-		Args:  cobra.MaximumNArgs(1),
-		Run: func(_ *cobra.Command, args []string) {
-			runTrees(args)
+		Use:   "trees",
+		Short: "Read the scan database and print the duplicate-tree report",
+		Args:  cobra.NoArgs,
+		Run: func(_ *cobra.Command, _ []string) {
+			runTrees()
 		},
 	}