Test both walk cancellation checks on their own (closes #81)

walkOneDir's check was hiding the worker's: a worker that walked a
queued directory on a cancelled scan still emitted nothing, because
walkOneDir stopped at its first entry. The worker test now queues a
missing directory, whose read fails and sends a warning before
walkOneDir's check is reached. A new test calls walkOneDir directly on
a cancelled scan and checks it returns no subdirectory to descend into.

Model: opus-5-5

TODO.md

@@ -32,9 +32,6 @@
 - a test fails when either walk cancellation check in `scan.go` is
   removed (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/81)
 
-- 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

@@ -157,11 +157,9 @@ func TestOpenReportDatabaseMissing(t *testing.T) {
 	}
 }
 
-func TestOpenDatabaseVersionMismatch(t *testing.T) {
+func TestOpenReportDatabaseVersionMismatch(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)
@@ -178,12 +176,7 @@ func TestOpenDatabaseVersionMismatch(t *testing.T) {
 
 	_, err = openReportDatabase(t.Context(), path)
 	if !errors.Is(err, errSchemaVersion) {
-		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)
+		t.Fatalf("err = %v, want errSchemaVersion", err)
 	}
 }