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).

internal/builder/builder.go

@@ -374,7 +374,7 @@ func build(ctx context.Context, run Runner, publish Publisher,
 		// only when asked — a check, a dry run, `build --ref` and `rebuild` never are. Its key is taken by the same
 		// steps a build takes, stopped before anything is built; what it takes is copied by digest.
 		if reuse != nil && reuse.keyFirst {
-			_, key, err := keyFor(ctx, run, publish, manifest, within, workspace, commit, credentials, artifacts, args,
+			keyed, key, err := keyFor(ctx, run, publish, manifest, within, workspace, commit, credentials, artifacts, args,
 				held, npmrcPath, npmrc, seatBases, src, say)
 			if err == nil && key.Key == "" {
 				err = fmt.Errorf("%w: %s", errNoKey, key.Why)
@@ -383,6 +383,11 @@ func build(ctx context.Context, run Runner, publish Publisher,
 				say("reuse", "not built for the merge: %v", err)
 				return Result{Commit: commit, NoKey: err.Error()}, err
 			}
+			// **A check keeps a build of the same key a verdict names** (novox/hq issue 703): asked again when only
+			// the trunk moved, it builds nothing the earlier check built of an unchanged source.
+			if kept := keptCheckBuild(ctx, *reuse, manifest.Module, key.Key, len(artifacts), say); kept != nil {
+				built, src, reused = kept.Made, keyed, kept
+			}
 		} else if reuse != nil && reuse.Scratch != nil {
 			taken, keyed, why := takeACheckBuild(ctx, run, publish, *reuse, manifest, within, workspace, commit,
 				credentials, artifacts, args, held, npmrcPath, npmrc, seatBases, src, onTrunk, say)
@@ -492,6 +497,32 @@ func takeACheckBuild(ctx context.Context, run Runner, publish Publisher, reuse R
 	return taken, keyed, ""
 }
 
+// keptCheckBuild is the build a check keeps instead of building (novox/hq issue 703): the one kept under the
+// module's key, when a verdict of the build seat names its key and every digest — the record alone is never trusted
+// (ADR 0350 §2) — and it holds every artifact the module declares. Nil, and the check builds, otherwise.
+func keptCheckBuild(ctx context.Context, reuse Reuse, module, key string, artifacts int,
+	say func(step, format string, args ...any)) *Reused {
+	if reuse.Scratch == nil || len(reuse.Named) == 0 {
+		return nil
+	}
+	found, ok, err := reuse.Scratch.Find(ctx, module, key)
+	switch {
+	case err != nil:
+		say("reuse", "the scratch namespace could not be read, so the check builds: %v", err)
+		return nil
+	case !ok:
+		return nil
+	case unnamed(reuse.Named, module, key, found) != "":
+		say("reuse", "a build is kept under %s and no verdict names it with its digests: built", twelve(key[3:]))
+		return nil
+	case len(found.Made) != artifacts:
+		return nil
+	}
+	kept := &Reused{From: found, Made: found.Made}
+	say("reuse", "kept, not built: %s", strings.Replace(kept.Words(), "reused from", "the build of", 1))
+	return kept
+}
+
 // keyFor takes a build's reuse key by the steps a build takes, each artifact stopped before anything is built or
 // published (ADR 0350 §2): its own copy of the source inputs, so the build's are untouched.
 func keyFor(ctx context.Context, run Runner, publish Publisher, manifest catalogue.Manifest,