Asking what the mesh would send must not change it

`status` hung. It composes a declaration for every node to answer *is
this machine running what I would send it*, and composing one assigns
each module a machine port — so the question wrote to the database, and
wrote to the same rows as the machine it was asking about.

`port_assignment` is unique on (node, machine). Two transactions
inserting the same port do not race, they queue: the second waits on the
index until the first commits. A status polled every two seconds while a
node applies is two writers on those rows, and the poll stopped
returning rather than returning something wrong — which is the better
failure of the two, and still a failure.

The latent version of this was there before anything polled: two
compositions running at once could both allocate.

So allocation belongs to the send path alone. The mesh chooses a port
when it commits to sending one; every other caller reads what was
chosen. A module with nothing assigned has never been sent, which is
precisely what "waiting" means — the read needs no number to be right
about that, and inventing one would make the answer worse.

Named rather than passed as a bare bool: at three call sites, `true` and
`false` say nothing about which of these two things is meant.

Checked by the lab, which now polls status throughout an apply.

cmd/mesh-control/push.go

@@ -265,7 +265,7 @@ func pushCommand(ctx context.Context, args []string) error {
 		// The private network is in here with everything else. It used to be composed separately
 		// and prepended, which meant every machine with an address was on it and no machine could
 		// be kept off. It is a module now, so it arrives the way a module does.
-		resources, err := declarationWith(ctx, open, n.Name, plan, settings, gens)
+		resources, err := declarationWith(ctx, open, n.Name, plan, settings, gens, Allocating)
 		if err != nil {
 			refusals = append(refusals, fmt.Sprintf("%s:\n%v", n.Name, err))
 			continue
@@ -335,7 +335,7 @@ func sendTo(ctx context.Context, open *stores, names []string) error {
 			refusals = append(refusals, fmt.Sprintf("%s:\n%v", name, err))
 			continue
 		}
-		resources, err := declarationWith(ctx, open, name, plan, settings, gens)
+		resources, err := declarationWith(ctx, open, name, plan, settings, gens, Allocating)
 		if err != nil {
 			refusals = append(refusals, fmt.Sprintf("%s:\n%v", name, err))
 			continue
@@ -399,7 +399,7 @@ func wouldSend(ctx context.Context, open *stores,
 		if err != nil {
 			continue
 		}
-		resources, err := declarationWith(ctx, open, n.Name, plan, settings, gens)
+		resources, err := declarationWith(ctx, open, n.Name, plan, settings, gens, Reading)
 		if err != nil {
 			continue
 		}