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/file.go

@@ -11,6 +11,7 @@ import (
 type fileArtifact []byte
 
 var _ KnownChecksum = new(fileArtifact)
+var _ CuresExempt = new(fileArtifact)
 
 // fileArtifactNamed embeds fileArtifact alongside a caller-supplied name.
 type fileArtifactNamed struct {
@@ -79,3 +80,6 @@ func (a *fileArtifact) Checksum() Checksum {
 func (a *fileArtifact) Cure(*RContext) (io.ReadCloser, error) {
 	return io.NopCloser(bytes.NewReader(*a)), nil
 }
+
+// CuresExempt exempts the cheap [KindFile] implementation.
+func (*fileArtifact) CuresExempt() {}