DLV-B9: a bundle answered in the registry seat's form keeps the artifact-store form it derives for each counted engine that does not read it

The builder's forms gain seat-reference for bundles and archives (oci-artifact-registry/<module>/<artifact>@<digest>);
the old-form window writes the older artifact-store reference beside it (catalogue.InArtifactStore), the oldest kept
in the single reference (ADR 0624 §1, §3), so mesh-build-go's seat-form answer no longer strands an engine stating nothing.

Claude-Session: https://claude.ai/code/session_01JG73KJNrH5EgFXMKKYSrig

sdk/forms/forms.go

@@ -32,20 +32,28 @@ const (
 	FormPackageVersion = "package-version"
 )
 
-// ArtifactForm is one reference form: its name, whether it is a first form, the artifact kinds it names, and how a
-// reference is told to be in it.
+// FormSeatReference is the form a registry seat answers for a bundle or an archive it published,
+// oci-artifact-registry/<module>/<artifact>@sha256:<digest> (novox/hq ADR 0605 §9; catalogue.SeatReference, read back by
+// catalogue.InRegistrySeat), read by a node-engine that states form:seat-reference (catalogue.ReferenceFormSeat). Not a
+// first form: an engine stating nothing reads its blob only in the older artifact-store form, which it derives (Older).
+const FormSeatReference = catalogue.ReferenceFormSeat
+
+// ArtifactForm is one reference form: its name, whether it is a first form, the artifact kinds it names, how a
+// reference is told to be in it, and, for a form that names the same artifact an older form does, that older reference
+// (Older, nil when it derives none): the old-form window writes it beside the answered one (ADR 0624 §1, §3).
 type ArtifactForm struct {
 	Name  string
 	First bool
 	Kinds []string
 	Is    func(reference string) bool
+	Older func(reference string) string
 }
 
 // ArtifactForms are the forms a builder knows, oldest first.
 type ArtifactForms []ArtifactForm
 
 // Current are the forms the builder writes and the assigner knows: the three first forms (delivery.BuiltArtifact's
-// reference by kind).
+// reference by kind), then the registry seat's form of a bundle or an archive.
 var Current = ArtifactForms{
 	{Name: FormArtifactStore, First: true, Is: func(r string) bool { return strings.HasPrefix(r, artifactStore) },
 		Kinds: []string{catalogue.ArtifactBundle, catalogue.ArtifactArchive, catalogue.ArtifactImage, catalogue.ArtifactUpstream}},
@@ -53,6 +61,16 @@ var Current = ArtifactForms{
 		Kinds: []string{catalogue.ArtifactImage, catalogue.ArtifactUpstream}},
 	{Name: FormPackageVersion, First: true, Is: func(r string) bool { return strings.LastIndex(r, "@") > 0 && !strings.Contains(r, "@sha256:") },
 		Kinds: []string{catalogue.ArtifactPackage}},
+	{Name: FormSeatReference, Kinds: []string{catalogue.ArtifactBundle, catalogue.ArtifactArchive},
+		Is: func(r string) bool {
+			seat, _, ok := catalogue.InRegistrySeat(r)
+			return ok && seat == catalogue.OciArtifactRegistrySeat
+		},
+		// The same blob in the artifact store's form: oci-artifact-registry/<m>/<a>@<d> is artifact-store://<m>/<a>/blobs/<d>.
+		Older: func(r string) string {
+			path, _ := catalogue.InArtifactStore(r)
+			return catalogue.ArtifactStoreScheme + path
+		}},
 }
 
 // FormNeed is the reader capability a form needs of a node-engine: `form:<name>`.
@@ -180,9 +198,12 @@ func (fs ArtifactForms) Kept(kind string, present, counted map[string][]string)
 // Choose names a built artifact by the references the build answered in the forms Kept keeps (ADR 0624 §1, §3): the
 // oldest kept form in the single Reference, every kept one in References when there are more than one. A reference in a
 // form Kept does not keep is left out, unless none of the answered is kept: then the artifact keeps what was answered,
-// its first answered in Reference, since a key names where its artifact is (ADR 0498 clause 1).
+// its first answered in Reference, since a key names where its artifact is (ADR 0498 clause 1). An answered reference
+// in a form that derives an older one (ArtifactForm.Older) answers that older one too, so the build answers one form and
+// the key keeps each a counted node reads.
 func (fs ArtifactForms) Choose(a delivery.BuiltArtifact, answered []string, present, counted map[string][]string) delivery.BuiltArtifact {
 	kept := fs.Kept(a.Kind, present, counted)
+	answered = fs.withOlder(a.Kind, answered)
 	type ref struct {
 		at        int
 		reference string
@@ -224,3 +245,22 @@ func (fs ArtifactForms) Choose(a delivery.BuiltArtifact, answered []string, pres
 	}
 	return a
 }
+
+// withOlder is answered with, after it, each older reference an answered one's form derives that was not answered itself.
+func (fs ArtifactForms) withOlder(kind string, answered []string) []string {
+	out := slices.Clone(answered)
+	for _, r := range answered {
+		name, ok := fs.FormOf(kind, r)
+		if !ok {
+			continue
+		}
+		for _, f := range fs {
+			if f.Name == name && f.Older != nil {
+				if older := f.Older(r); !slices.Contains(out, older) {
+					out = append(out, older)
+				}
+			}
+		}
+	}
+	return out
+}