Keep a build node's pause in a pauses state of its own, so the steps state can drop its old keys without ending a pause

The steps state of the build modules filled to the bus's 64 MiB cap (27,370 keys of mesh-build-go,
none ever removed) and refused a passed check's end. Its keys get an age in the catalogue; the
pause key lived among them and would then have ended by itself. Puller.PausePut writes it to
delivery.PausesState instead; Put stays the fallback for a module not yet moved.

go/buildstep/pull.go

@@ -65,10 +65,13 @@ type Puller struct {
 	PullWait  time.Duration
 	Heartbeat time.Duration
 	Logf      func(format string, args ...any)
-	// Load reads one key of the module's steps state (nil and no error when it is absent). When set, Pull reads the
-	// node's pause key (delivery.PauseKey) before it pulls, and Pause and Resume write it with Put, so a pause outlives
-	// a restart.
+	// Load reads one key of the state the node's pause is kept in (nil and no error when it is absent). When set, Pull
+	// reads the node's pause key (delivery.PauseKey) before it pulls, and Pause and Resume write it with PausePut, so a
+	// pause outlives a restart.
 	Load func(key string) ([]byte, error)
+	// PausePut writes the node's pause key: the module's pauses state (delivery.PausesState), which has no age, so a
+	// pause never ends by itself while the steps state drops its old keys. Put when nil.
+	PausePut Put
 	// WatchConfig, when set, follows the module's config value `capacity` (CapacitySetting) as mesh-config resolves it
 	// for this node, and wakes the puller on each change (watchCapacity): WatchConfigServed in a module, a test's own
 	// double. Until it hands a value the puller judges by Capacity.
@@ -121,7 +124,11 @@ func (p *Puller) setPaused(paused bool, by string) error {
 		if err != nil {
 			return err
 		}
-		if _, err := p.Put(key, json.RawMessage(raw)); err != nil {
+		put := p.PausePut
+		if put == nil {
+			put = p.Put
+		}
+		if _, err := put(key, json.RawMessage(raw)); err != nil {
 			return fmt.Errorf("buildstep: writing the pause of %s: %w", p.Node, err)
 		}
 	}