Leave an eligible index for a confirmed collect instead of letting it go alone

Let go of alone after a build, an index's platforms stayed for ever under a record that
said collected, so no later collect could reach them (re-review of #144).

cmd/mesh-controller/collect.go

@@ -59,6 +59,9 @@ type sweepResult struct {
 	LetGo []string
 	// Skipped is how many references the sweep will not address (novox/hq issue 226).
 	Skipped int
+	// Indexes is how many eligible indexes a sweep a person did not confirm left for one that is: an
+	// index goes only with its platform manifests (novox/hq ADR 0257 §4).
+	Indexes int
 	// Left is how many eligible references were not asked about this time.
 	Left int
 	// Bounded is whether it stopped at its bounds rather than at a refusal.
@@ -85,7 +88,7 @@ func sweep(ctx context.Context, inv *inventory.Inventory, store artifacts.Store,
 	// 0257), and a kept index keeps its own: which platform manifests the kept indexes name is read
 	// from the store for every repository the sweep will touch, before the first delete. A single
 	// read that fails stops the sweep before it deletes anything, as a kept archive that cannot be
-	// held does. The sweep after a build lets an index go alone, as it always has.
+	// held does. The sweep after a build leaves an eligible index for such a collect, below.
 	store.Spare = nil
 	if bounds.platforms {
 		if inv == nil {
@@ -124,6 +127,22 @@ func sweep(ctx context.Context, inv *inventory.Inventory, store artifacts.Store,
 			}
 			break
 		}
+		if !bounds.platforms {
+			// **An index waits for a confirmed collect** (novox/hq ADR 0257 §4). Let go of alone,
+			// its platform manifests would stay in the store and the record would say collected, so
+			// no later sweep would offer it again and they would stay for ever. Left eligible, the
+			// next collect a person confirms takes it with them.
+			index, err := store.IsIndex(within, reference)
+			if err != nil && !errors.Is(err, artifacts.Gone) && !errors.Is(err, artifacts.ErrNotOurs) {
+				r.Stopped = fmt.Sprintf("the artifact store could not say whether %s is an index, so nothing more was asked of it: %v", reference, err)
+				r.Left = len(references) - i
+				break
+			}
+			if index {
+				r.Indexes++
+				continue
+			}
+		}
 		err := store.LetGo(within, reference)
 		if err == nil || errors.Is(err, artifacts.Gone) {
 			// Gone is the outcome wanted, already true. Recorded so the next sweep does not ask
@@ -215,6 +234,9 @@ func collect(ctx context.Context, inv *inventory.Inventory) {
 	if r.Skipped > 0 {
 		fmt.Fprintf(os.Stderr, "%d artifact(s) the sweep will not address were skipped\n", r.Skipped)
 	}
+	if r.Indexes > 0 {
+		fmt.Fprintf(os.Stderr, "%d eligible index(es) wait for a collect a person confirms, which takes their platforms too\n", r.Indexes)
+	}
 }
 
 // holdKept holds every kept archive by its manifest, stopping at the first refusal by the store.