Make atomic file replacement crash durable

internal/fileops/sync_directory_unix.go

@@ -4,6 +4,7 @@ package fileops
 
 import (
 	"errors"
+	"fmt"
 	"os"
 	"syscall"
 )
@@ -16,10 +17,10 @@ func syncDirectory(path string) error {
 	defer func() { _ = directory.Close() }()
 
 	err = directory.Sync()
-	// Some Unix filesystems do not implement directory syncing. Only their
-	// explicit unsupported-operation errors are safe to treat as best effort.
+	// Some Unix filesystems do not implement directory syncing. Report this
+	// explicitly so callers do not confuse visible replacement with a durable one.
 	if errors.Is(err, syscall.EINVAL) || errors.Is(err, syscall.ENOTSUP) {
-		return nil
+		return fmt.Errorf("%w for %q: %w", ErrDirectorySyncUnsupported, path, err)
 	}
 	return err
 }