Build a change once: the check builds what its merge moves, the walk takes it by key

A merge built every moved module a second time although its check had judged
the same tree (issue 649). The check now builds into the scratch namespace
under a reuse key (source fingerprint plus the builder's own digest), and the
walk of the trunk commit copies a build of the same key by digest instead of
building, saying per module why when it cannot (novox/hq ADR 0350).

cmd/mesh-controller/main.go

@@ -104,6 +104,8 @@ func run() error {
 		return replayCommand(ctx, args[1:])
 	case "kill":
 		return killCommand(ctx, args[1:])
+	case "forget-checks":
+		return forgetChecksCommand(ctx, args[1:])
 	case "pause", "resume":
 		return pauseCommand(ctx, args[0], args[1:])
 	case "collection":
@@ -429,8 +431,14 @@ func (b builds) Built(ctx context.Context, result link.BuildResult) error {
 	}
 	fmt.Printf("%s: %s %s registered, built on %s from %s\n",
 		result.ID, manifest.Module, manifest.Version, result.On, short(result.Commit))
+	// What the walk's build did, kept on the walk whose ask it answers (novox/hq ADR 0350 §4); a build seat from
+	// before it says nothing, and that is what is kept.
+	made := result.MadeWords()
+	if result.Reused != "" || result.BuiltWhy != "" {
+		fmt.Printf("%s: %s %s\n", result.ID, manifest.Module, made)
+	}
 	saysWhenThePolicyActs(ctx, b.inv, manifest.Module)
-	planBuilt(ctx, b.open, manifest.Module, result.Commit, "", asked, result.ID)
+	planBuiltSaying(ctx, b.open, manifest.Module, result.Commit, "", asked, result.ID, made)
 	// A module registered may be one a machine is now behind: `status` is composed again.
 	statusFrom.nudge()
 	return nil