internal/pkg: optionally exempt implementations from cures counter

This avoids holding up many slots with a long pipeline.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/tar.go

@@ -16,6 +16,8 @@ type tarArtifact struct {
 	f Artifact
 }
 
+var _ CuresExempt = new(tarArtifact)
+
 // tarArtifactNamed embeds tarArtifact for a [fmt.Stringer] tarball.
 type tarArtifactNamed struct {
 	tarArtifact
@@ -211,3 +213,7 @@ func (a *tarArtifact) Cure(t *TContext) (err error) {
 	}
 	return
 }
+
+// CuresExempt exempts the cheap [KindTar] implementation often at the end of a
+// [FileArtifact] pipeline.
+func (*tarArtifact) CuresExempt() {}