Bring main into release 57's branch, conflicts resolved

So the release branch carries what main gained, and the store-tests drop check passes again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/builder/reuse.go

@@ -257,6 +257,13 @@ func (s scratchPublisher) PublishArchive(ctx context.Context, repository string,
 	return s.to.PublishArchive(ctx, ScratchPrefix+repository, body, digest)
 }
 
+func (s scratchPublisher) LendBases(ctx context.Context, repository string, bases []string) (int, error) {
+	if l, can := s.to.(BaseLender); can {
+		return l.LendBases(ctx, ScratchPrefix+repository, bases)
+	}
+	return 0, nil
+}
+
 func (s scratchPublisher) MirrorImage(ctx context.Context, from, repository string) (string, error) {
 	m, can := s.to.(Mirrorer)
 	if !can {