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).

cmd/mesh-builder/main.go

@@ -311,6 +311,16 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 			for _, p := range request.Reuse.Pulls {
 				reuse.Pulls = append(reuse.Pulls, builder.PullRef{Owner: p.Owner, Repo: p.Repo, Number: p.Number})
 			}
+			// Only what a verdict names is taken (ADR 0350 §2).
+			for _, b := range request.Reuse.Builds {
+				named := builder.NamedBuild{Pull: builder.PullRef{Owner: b.Pull.Owner, Repo: b.Pull.Repo,
+					Number: b.Pull.Number}, Check: b.Check, Module: b.Built.Module, Key: b.Built.Key,
+					Digests: map[string]string{}}
+				for _, m := range b.Built.Made {
+					named.Digests[m.Name] = builder.DigestOf(m.Reference)
+				}
+				reuse.Named = append(reuse.Named, named)
+			}
 			built, err = builder.BuildReusing(building, builder.Labelled(builder.Command, request.ID), publisher, reuse,
 				request.Repository, request.Path, request.Ref, workspace, request.Held, npmrc,
 				forgeFrom(), say, request.Seats)