Act under a lease, keep accounts by order, one writer at composition (hq to-be 45 Phase 2)

Two controllers could both act (issue 204), a reconcile's report could
overtake the apply after it and the digest decided (issue 267), and a grant
could make a second writer of a machine's report.

- The lease (internal/lease, ADR 0229): mesh-controller_lease key `holder`,
  15 s age, renewed every 5 s by compare-and-set; the epoch is the revision
  it was taken at. The gate is the clock (stops 3 s before expiry); a refused
  renewal is a loss and the process exits; a holder that stops gives it back.
  serve takes it before asserting the bus. Epochs kept in the store
  (migration 0068 controller_epoch) as a floor: a bucket raised from nothing
  is compacted past it. Unleased (no epoch, S12 urgent) only when nobody
  holds it and the bus will not let it be written. A shell command acts
  under the holder's epoch, or its own lease when none.
- Declarations carry `epoch` inside the signed envelope, only to a machine
  whose latest account carried a report_sequence (mesh-host #35); would-send
  is composed with the epoch last sent. Allot and the send both pass the gate.
- Reports: contract in internal/link/order.go (epoch, sequence,
  report_sequence, older_than, refused_older). Accounts kept by epoch, then
  sequence, then report sequence; older refused, counted; unordered reports
  keep the digest rule. Plans by compare-and-set on a revision, with epoch.
  Conditions and calls carry the epoch and are not written off the lease.
- S12 and S13 (naming the writer by epoch) watched, D5 run; reset of the
  bucket said. Writers table compiled in and enforced in PermissionsFor; the
  controller no longer publishes mesh.control.>. A contract per consumed
  kind, and the empty-on-error lint over the repository.
- mesh-host pinned to its main with the epoch in the validator (D1 validates
  the envelope as sent).

Needs mesh-host's genesis lock with the lease grant (mesh-host PR) for
TestTheInstallersFirstUserListIsWhatTheControllerWouldCompose.

cmd/mesh-controller/upgrades.go

@@ -383,13 +383,13 @@ func (f following) SourceMoved(ctx context.Context, m link.SourceMoved) error {
 		fmt.Printf("  the last tier depends on itself: %s — built together, in no order\n",
 			strings.Join(plan.Tiers[len(plan.Tiers)-1], ", "))
 	}
-	if err := inv.SavePlan(ctx, plan); err != nil {
+	if err := inv.SavePlan(ctx, &plan); err != nil {
 		return notNow(err)
 	}
 	// Closed after the newer plan is kept, never before: a controller replaced between the two leaves
 	// both open, which the next merge settles, rather than neither.
 	for _, old := range superseded {
-		if err := inv.SavePlan(ctx, old); err != nil {
+		if err := inv.SavePlan(ctx, &old); err != nil {
 			return notNow(err)
 		}
 		fmt.Printf("  %s (%s at %s) is %s\n", old.ID, old.Repository, short(old.Commit), old.Note)
@@ -411,7 +411,7 @@ func (f following) SourceMoved(ctx context.Context, m link.SourceMoved) error {
 	if err := askTier(ctx, inv, &plan); err != nil {
 		return notNow(err)
 	}
-	if err := inv.SavePlan(ctx, plan); err != nil {
+	if err := inv.SavePlan(ctx, &plan); err != nil {
 		return notNow(err)
 	}
 	return nil