Keep a check build whose key is unchanged instead of building it again

A fresh check asked when only the trunk moved rebuilt every image, though
the reuse key said nothing it is built from changed: a controller check
took about 12 minutes instead of 5.5. The check now keeps a build kept
under its key when a verdict of the build seat names that key and every
digest, names it as the build's own check so a walk takes it, and the
cleanup keeps it while any check names it (novox/hq issue 703).

cmd/mesh-controller/checks.go

@@ -273,6 +273,7 @@ func checkRequestFor(ctx context.Context, open *stores, p link.PullUpdated, scop
 			}
 		}
 	}
+	builds := checkBuildsOf(p, scope, entries, clone)
 	return link.BuildRequest{
 		ID:         link.NewBuildID(time.Now()),
 		Repository: repository,
@@ -282,10 +283,33 @@ func checkRequestFor(ctx context.Context, open *stores, p link.PullUpdated, scop
 		Source:     sourceOnSeat(source),
 		Check: &link.CheckRequest{Owner: p.Owner, Repo: p.Repo, Number: p.Number, Base: p.Base,
 			Paths: p.Paths, Beside: beside, Modules: scope.Modules, Dependents: scope.Dependents, New: scope.New,
-			Manifests: scope.Manifests, Judge: scope.Judge, Builds: checkBuildsOf(p, scope, entries, clone)},
+			Manifests: scope.Manifests, Judge: scope.Judge, Builds: builds, Named: namedForTheCheck(ctx, builds)},
 	}, nil
 }
 
+// namedForTheCheck are the builds the seat's verdicts name for the modules a check builds, read once for the check
+// (novox/hq issue 703): what it may keep instead of building again. None when they cannot be read, and the check
+// builds, as before.
+func namedForTheCheck(ctx context.Context, builds []link.CheckBuild) []link.CheckedBuild {
+	var modules []string
+	for _, b := range builds {
+		if b.Skip == "" {
+			modules = append(modules, b.Module)
+		}
+	}
+	if len(modules) == 0 {
+		return nil
+	}
+	named, err := checkedBuildsOfModules(ctx, modules)
+	if err != nil {
+		fmt.Printf("  the checks' builds of %d module(s) could not be read, so the check builds them: %v\n",
+			len(modules), err)
+		// empty-on-error: said above; a check handed no named build builds every module, as before issue 703
+		return nil
+	}
+	return named
+}
+
 // checkBuildsOf are the modules a merge of the change would build, in the walk's order, as the check builds them
 // for the walk to take (novox/hq ADR 0350 §1): a module built from the repository checked, at the branch it merges
 // into, from the merge the check makes; any other — a module standing on one — from its own repository at the