Hold every kept archive by a manifest so the store's collector keeps it (hq issue 253)

The store's garbage-collect marks only from manifests, and archives were
published as bare blobs, so the first real collection would delete every
archive the mesh keeps. PublishArchive now puts a deterministic OCI holder
manifest (empty config, one layer) beside each archive; the sweep holds every
kept archive before it lets anything go, which backfills existing bare blobs,
and lets go of an archive holder-first. A forgotten module no longer keeps its
five recent builds (ADR 0189). `collection [--json]` reports kept archives
held/unheld and what may be let go, so the dry run can be lifted on evidence.

internal/artifacts/store_test.go

@@ -20,6 +20,17 @@ func fakeStore(t *testing.T, answer int) (Store, *[]string) {
 	t.Helper()
 	var asked []string
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method == http.MethodHead && strings.Contains(r.URL.Path, "/blobs/") {
+			// An archive's size, asked so its holder can be named (novox/hq issue 253). A store
+			// that does not have the thing does not have its blob either.
+			if answer == http.StatusNotFound {
+				w.WriteHeader(http.StatusNotFound)
+				return
+			}
+			w.Header().Set("Content-Length", "7")
+			w.WriteHeader(http.StatusOK)
+			return
+		}
 		if r.Method != http.MethodDelete {
 			t.Errorf("the store was asked %s %s; collecting is a delete", r.Method, r.URL.Path)
 		}
@@ -45,8 +56,14 @@ func TestAnImageAndAnArchiveAreAskedForAtTheirOwnEndpoints(t *testing.T) {
 	if err := store.LetGo(ctx, archive); err != nil {
 		t.Fatal(err)
 	}
-	want := []string{"/v2/web/app/manifests/sha256:abc123", "/v2/web/config/blobs/sha256:def456"}
-	if len(*asked) != 2 || (*asked)[0] != want[0] || (*asked)[1] != want[1] {
+	// The archive's holder goes first, then the archive (novox/hq issue 253).
+	_, holder := Holder("sha256:def456", 7)
+	want := []string{
+		"/v2/web/app/manifests/sha256:abc123",
+		"/v2/web/config/manifests/" + holder,
+		"/v2/web/config/blobs/sha256:def456",
+	}
+	if strings.Join(*asked, " ") != strings.Join(want, " ") {
 		t.Fatalf("the store was asked %v; want %v", *asked, want)
 	}
 }