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-builder/holder.go

@@ -41,6 +41,9 @@ type holder struct {
 	say func(link.HolderState) error
 	// remove runs what a kill needs outside the build: the containers left behind.
 	remove builder.Runner
+	// scratch is where check builds are kept (novox/hq ADR 0350 §5): what the week's sweep lets go of. Nil where
+	// this machine publishes to no artifact store.
+	scratch builder.Scratch
 
 	mu      sync.Mutex
 	paused  bool
@@ -131,6 +134,34 @@ func (h *holder) stateWhilePaused(ctx context.Context, every time.Duration) {
 	}
 }
 
+// expireChecks lets go of every check build older than a week (novox/hq ADR 0350 §5), at start and every while
+// after. Every holder sweeps the one store; what one let go of, another finds gone.
+func (h *holder) expireChecks(ctx context.Context, every time.Duration, now func() time.Time) {
+	sweep := func() {
+		if h.scratch == nil {
+			return
+		}
+		n, err := h.scratch.Expire(ctx, builder.CheckBuildsKept, now())
+		switch {
+		case err != nil:
+			fmt.Fprintf(os.Stderr, "check builds older than %s: %d let go of, and not all: %v\n", builder.CheckBuildsKept, n, err)
+		case n > 0:
+			fmt.Fprintf(os.Stderr, "check builds older than %s: %d check(s)' builds let go of\n", builder.CheckBuildsKept, n)
+		}
+	}
+	sweep()
+	tick := time.NewTicker(every)
+	defer tick.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-tick.C:
+			sweep()
+		}
+	}
+}
+
 // begin records the build this machine took, with the cancel that ends it.
 func (h *holder) begin(request link.BuildRequest, cancel context.CancelFunc) *running {
 	r := &running{request: request, started: time.Now(), cancel: cancel, done: make(chan struct{})}