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).

internal/inventory/mirrored_test.go

@@ -179,3 +179,33 @@ func TestTheCopiesTheRecordsAlreadyNameAreRecorded(t *testing.T) {
 		t.Fatalf("recorded %s by %s", reference, build)
 	}
 }
+
+// A copy let go of and copied again by a later build is held again: its record is back, and the sweep
+// decides about it as before.
+func TestACopyLetGoOfAndCopiedAgainIsHeldAgain(t *testing.T) {
+	inv := fresh(t)
+	ctx := context.Background()
+	holding(t, inv, "proxy")
+	golang := mirrorRef("upstream/docker.io/library/golang", 1)
+	builtOn(t, inv, "b01", "proxy", 1, []string{golang}, golang)
+	if err := inv.MarkCollected(ctx, []string{golang}); err != nil {
+		t.Fatal(err)
+	}
+	if s := stateOf(t, inv, golang); s.State != ArtifactCollected {
+		t.Fatalf("a let-go copy reads %+v", s)
+	}
+	builtOn(t, inv, "b02", "proxy", 2, []string{golang}, golang)
+	if s := stateOf(t, inv, golang); s.State != ArtifactKept || !slices.Equal(s.Why, []string{KeptStoodOn}) {
+		t.Fatalf("a copy copied again reads %+v; want kept again", s)
+	}
+	// A person's record never undoes a collection: only a build that copied it says it is there.
+	if err := inv.MarkCollected(ctx, []string{golang}); err != nil {
+		t.Fatal(err)
+	}
+	if err := inv.RecordMirrored(ctx, "", "again", []string{golang}); err != nil {
+		t.Fatal(err)
+	}
+	if s := stateOf(t, inv, golang); s.State != ArtifactCollected {
+		t.Fatalf("a person's record brought a collected copy back: %+v", s)
+	}
+}