J4 (J3 item 7): merge A3's wip/registry-a3-sdk (c974a11): a holder's receiving side of a file in parts joins its sender

Brings A3's parts receiver and bus adapters, the shared caller lists and builder versions question, a transfer read
back from bus state (0424e87) and catalogue.SeatReference (c10ef07). c974a11 adds nothing: its version_numbered.go
and BuildAnswer.OutOfStep are the join's own. publish stays out (ADR 0627, 1a149d4): packages_list_test keeps the
join's form. TestAHolderTakesAFileInParts is a subtest of TestAFileTravelsInTheParts, so unit tests stay at 476.

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

go/catalogue/artifacts.go

@@ -79,6 +79,13 @@ const (
 	ProvisionerSeat = contractseats.MeshProvisioner
 )
 
+// SeatReference is the reference a build names an image, bundle, archive or upstream copy by once it published it
+// through a registry seat (novox/hq ADR 0605 §9): `<seat>/<module>/<artifact>@sha256:<digest>`, as InRegistrySeat reads
+// it back. The store's address is left out, as in the older form: a reader routes it (Routed).
+func SeatReference(seat, module, artifact, digest string) string {
+	return seat + "/" + module + "/" + artifact + "@" + digest
+}
+
 // InRegistrySeat reads a reference in the form a registry seat answers, `<seat>/<module>/<artifact>@sha256:<digest>`:
 // the seat it came through and the rest. A seat's name holds no dot or port, so no image of an outside registry reads
 // as one, and a kept `artifact-store://` reference is not one.

go/catalogue/artifacts_test.go

@@ -42,4 +42,11 @@ func TestAReferenceIsRecordedWithoutTheStoresAddress(t *testing.T) {
 			t.Errorf("Routed(%q) = %q, want %q", reference, got, want)
 		}
 	}
+	// A build module names what it published in the seat's form, and reads it back as the seat it came through.
+	for _, seat := range RegistrySeats {
+		ref := SeatReference(seat, "gitea", "config", digest)
+		if got, path, ok := InRegistrySeat(ref); !ok || got != seat || path != "gitea/config@"+digest {
+			t.Errorf("SeatReference(%s) = %q, read back as %q %q %v", seat, ref, got, path, ok)
+		}
+	}
 }