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/push.go

@@ -249,7 +249,13 @@ func serve(ctx context.Context) (err error) {
 	// (novox/hq to-be 45 §9).
 	checkEvents = bus
 	// And every walk this controller keeps in a new state, for the delivery it walks (novox/hq ADR 0239).
-	inventory.PlanSaved = func(_ *inventory.Inventory, p inventory.Plan) { sayPlanMoved(ctx, bus, open.inventory, p) }
+	inventory.PlanSaved = func(_ *inventory.Inventory, p inventory.Plan) {
+		sayPlanMoved(ctx, bus, open.inventory, p)
+		// A walk ended has taken what it reuses: its merges' check builds go (novox/hq ADR 0350 §5).
+		if p.WalkSaid == inventory.WalkEnded {
+			go forgetChecksOfWalk(context.WithoutCancel(ctx), open.inventory, p)
+		}
+	}
 	if err := server.Checks(following{open}); err != nil {
 		return err
 	}