Let platform manifests go only on a confirmed collect, and copy again what a sweep took

An unrecorded index or a copy in progress can name a platform the records do not see, so
only a person's collect, after its dry run, takes an index's platforms, and only once every
kept index of each repository it touches was read. A copy missing a platform is copied
again, and a copy a build holds again is no longer recorded as collected (review of #144).

cmd/mesh-controller/mirrors.go

@@ -64,7 +64,9 @@ type mirrorsAnswer struct {
 	} `json:"counts"`
 	// Recorded, AlreadyRecorded and Refused answer a record: what was (or, a dry run, would be)
 	// recorded, what the records already held, and what was refused, with why.
-	Recorded        []string          `json:"recorded,omitempty"`
+	Recorded []string `json:"recorded,omitempty"`
+	// Then says what recording does: a recorded copy is eligible, and the next sweep lets it go.
+	Then            string            `json:"then,omitempty"`
 	AlreadyRecorded []string          `json:"already_recorded,omitempty"`
 	Refused         map[string]string `json:"refused,omitempty"`
 }
@@ -132,6 +134,15 @@ func recordMirrors(ctx context.Context, inv *inventory.Inventory, store artifact
 		record = append(record, reference)
 	}
 	a.Recorded = record
+	if len(record) > 0 {
+		verb := "would become"
+		if real {
+			verb = "became"
+		}
+		a.Then = fmt.Sprintf("%d cop%s %s eligible: the next sweep (after any build, or collect) lets each go "+
+			"unless one of the five kept builds of a module the mesh holds stood on it", len(record),
+			map[bool]string{true: "y", false: "ies"}[len(record) == 1], verb)
+	}
 	if real && len(record) > 0 {
 		if err := inv.RecordMirrored(ctx, "", why, record); err != nil {
 			return a, fmt.Errorf("recording %d copies: %w", len(record), err)
@@ -188,14 +199,16 @@ func mirrorsCommand(ctx context.Context, args []string) error {
 		if err != nil {
 			return err
 		}
-		if *confirm {
-			f.record(ctx, "mirrors", []string{"--record", *record})
-		}
 		answer, err = recordMirrors(ctx, inv, artifacts.Store{Address: address}, splitReferences(*record), known,
 			strings.TrimSpace(*f.why), *confirm)
 		if err != nil {
 			return err
 		}
+		// The hand act is logged once the records say what it did, never before: an act that
+		// failed half-way is not logged as done.
+		if *confirm && len(answer.Recorded) > 0 {
+			f.record(ctx, "mirrors", append([]string{"--record"}, answer.Recorded...))
+		}
 	}
 	if *asJSON {
 		encoder := json.NewEncoder(os.Stdout)
@@ -205,6 +218,9 @@ func mirrorsCommand(ctx context.Context, args []string) error {
 	if answer.DryRun && len(answer.Recorded) > 0 {
 		fmt.Println("a dry run: nothing was recorded (--confirm --why <text> to record)")
 	}
+	if answer.Then != "" {
+		fmt.Println(answer.Then)
+	}
 	for _, r := range answer.Recorded {
 		fmt.Printf("  record   %s\n", r)
 	}