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/compress.go

@@ -25,6 +25,7 @@ type decompressArtifact struct {
 }
 
 var _ FileArtifact = new(decompressArtifact)
+var _ CuresExempt = new(decompressArtifact)
 
 // decompressArtifactNamed embeds decompressArtifact for a [fmt.Stringer] stream.
 type decompressArtifactNamed struct {
@@ -117,3 +118,7 @@ func (a *decompressArtifact) Cure(r *RContext) (io.ReadCloser, error) {
 		return nil, os.ErrInvalid
 	}
 }
+
+// CuresExempt exempts the cheap [KindDecompress] implementation often part of
+// a [FileArtifact] pipeline.
+func (*decompressArtifact) CuresExempt() {}