Make atomic file replacement crash durable

internal/fileops/sync_directory_windows.go

@@ -4,6 +4,7 @@ package fileops
 
 import (
 	"errors"
+	"fmt"
 
 	"golang.org/x/sys/windows"
 )
@@ -29,12 +30,12 @@ func syncDirectory(path string) error {
 
 	err = windows.FlushFileBuffers(directory)
 	// Windows filesystems may reject flushing a directory handle even when it
-	// was opened correctly. Preserve every error except the documented forms
-	// that mean this operation is unavailable for the handle or filesystem.
+	// was opened correctly. Report documented unavailable forms explicitly so
+	// callers do not confuse visible replacement with a durable one.
 	if errors.Is(err, windows.ERROR_INVALID_FUNCTION) ||
 		errors.Is(err, windows.ERROR_INVALID_HANDLE) ||
 		errors.Is(err, windows.ERROR_NOT_SUPPORTED) {
-		return nil
+		return fmt.Errorf("%w for %q: %w", ErrDirectorySyncUnsupported, path, err)
 	}
 	return err
 }