Take only a check build its verdict names, and keep a build another check still names

The artifact store takes writes from anything on its network, so a record
planted under another merge's key could have been registered: the walk now
takes a scratch build only when the build seat's own verdict for one of its
pull requests names its key and digests. Deleting by digest also took builds
another check shared (review of #311; novox/hq ADR 0350).

internal/link/queue.go

@@ -364,6 +364,64 @@ func ReadBuildEvents(ctx context.Context, js *broker.JetStream, seat string, sin
 	return heard, nil
 }
 
+// ReadCheckBuilds is every build the seat's checks of those pull requests said they made for their merges since a
+// moment (novox/hq ADR 0350 §2): read from the seat's own outcomes on the events stream, which only its holders may
+// publish, never from the artifact store. Newest first.
+func ReadCheckBuilds(ctx context.Context, js *broker.JetStream, seat string, pulls []PullRef, since time.Time) ([]CheckedBuild, error) {
+	if len(pulls) == 0 {
+		return nil, nil
+	}
+	wanted := map[PullRef]bool{}
+	for _, p := range pulls {
+		wanted[p] = true
+	}
+	subject := BuildOutcomeOf(seat)
+	sub, err := js.Context().PullSubscribe(subject, "",
+		nats.BindStream(broker.EventsStream), nats.StartTime(since), nats.AckNone())
+	if err != nil {
+		return nil, fmt.Errorf("cannot read %s from the bus: %w", subject, err)
+	}
+	defer func() { _ = sub.Unsubscribe() }()
+	info, err := sub.ConsumerInfo()
+	if err != nil {
+		return nil, fmt.Errorf("cannot read %s from the bus: %w", subject, err)
+	}
+	var out []CheckedBuild
+	for remaining := info.NumPending; remaining > 0; {
+		batch, err := sub.Fetch(int(min(remaining, 500)), nats.MaxWait(2*time.Second))
+		if err != nil && !errors.Is(err, nats.ErrTimeout) && !errors.Is(err, context.DeadlineExceeded) {
+			return nil, fmt.Errorf("reading %s: %w", subject, err)
+		}
+		if len(batch) == 0 {
+			break
+		}
+		remaining -= uint64(len(batch))
+		for _, msg := range batch {
+			var r BuildResult
+			if json.Unmarshal(msg.Data, &r) != nil || r.Checked == nil || r.Check == nil || r.Failed != "" {
+				continue
+			}
+			pull := PullRef{Owner: r.Checked.Owner, Repo: r.Checked.Repo, Number: r.Checked.Number}
+			if !wanted[pull] {
+				continue
+			}
+			for _, b := range r.Check.Built {
+				if b.NotBuilt == "" && b.Key != "" {
+					out = append(out, CheckedBuild{Pull: pull, Check: r.ID, Built: b})
+				}
+			}
+		}
+		if ctx.Err() != nil {
+			return nil, ctx.Err()
+		}
+	}
+	// Newest first: the stream's order is the order they were said.
+	for i, j := 0, len(out)-1; i < j; i, j = i+1, j-1 {
+		out[i], out[j] = out[j], out[i]
+	}
+	return out, nil
+}
+
 // --- the cancelled set ----------------------------------------------------------------------
 
 // safeKey is an id that can be a key, and a subject token, as it is: a build id, never a pattern.