Test that scan refuses another schema version (closes #64)

The version-mismatch test only opened its database through
openReportDatabase, so nothing exercised the branch of initSchema that
stops scan on a database stamped with an unknown schema version. The
test now opens the same database through openScanDatabase too and
requires errSchemaVersion, and is renamed to match the unversioned-file
test beside it, which also covers both paths.

Model: opus-5-5

TODO.md

@@ -29,6 +29,9 @@
 
 # Completed Steps
 
+- test that `scan` refuses a database with another schema version
+  (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/64)
+
 - correct four inaccurate comments in `cancel_test.go` and rename
   `walkCancelInFlightDirs` to `walkCancelInFlightFiles` (2026-10-04,
   https://git.eeqj.de/sneak/sfdupes/issues/33)

db_test.go

@@ -157,9 +157,11 @@ func TestOpenReportDatabaseMissing(t *testing.T) {
 	}
 }
 
-func TestOpenReportDatabaseVersionMismatch(t *testing.T) {
+func TestOpenDatabaseVersionMismatch(t *testing.T) {
 	t.Parallel()
 
+	// A database stamped with a schema version other than 0 and
+	// schemaVersion. report, trees and scan must all refuse it.
 	path := testDBPath(t)
 
 	db, err := openScanDatabase(t.Context(), path)
@@ -176,7 +178,12 @@ func TestOpenReportDatabaseVersionMismatch(t *testing.T) {
 
 	_, err = openReportDatabase(t.Context(), path)
 	if !errors.Is(err, errSchemaVersion) {
-		t.Fatalf("err = %v, want errSchemaVersion", err)
+		t.Fatalf("report: err = %v, want errSchemaVersion", err)
+	}
+
+	_, err = openScanDatabase(t.Context(), path)
+	if !errors.Is(err, errSchemaVersion) {
+		t.Fatalf("scan: err = %v, want errSchemaVersion", err)
 	}
 }