Let the build modules follow their capacity on the bus, so a node raised from capacity 0 starts pulling (issue 1887)

g14's Go builder sat idle at capacity 2 because nothing woke its parked
puller. mesh-build-go, mesh-build-node and mesh-build-image now pin mesh-sdk
b9911dc1, whose puller watches the config value capacity as mesh-config
resolved it and wakes on each change, and hand it that watch.

modules/mesh-build-node/cmd/mesh-build-node/asks.go

@@ -45,8 +45,12 @@ func (m *Module) Puller(steps []StepTool, source buildstep.AskSource) *buildstep
 		source = buildstep.SeatAsks{Language: delivery.AskNode}
 	}
 	capacity := m.Capacity
+	// The capacity's own change wakes the puller (issue 1887): the runtime's watch of the config value as mesh-config
+	// resolved it; the settings file is read only when that watch cannot start. A test's Capacity is its own.
+	var watch func(string, func(stdio.ProvisionChange) error) error
 	if capacity == nil {
 		capacity = buildstep.NodeCapacity(m.Settings, buildstep.DefaultCapacities, m.Node, m.Say)
+		watch = buildstep.WatchConfigServed
 	}
 	ask := m.askSteps(steps)
 	var load func(string) ([]byte, error)
@@ -54,7 +58,7 @@ func (m *Module) Puller(steps []StepTool, source buildstep.AskSource) *buildstep
 		load = func(key string) ([]byte, error) { return m.ReadSteps.Get(key) }
 	}
 	return &buildstep.Puller{Source: source, Language: delivery.AskNode, Node: m.Node, Put: m.Steps, Load: load,
-		Capacity: capacity, Logf: m.Say,
+		Capacity: capacity, WatchConfig: watch, Logf: m.Say,
 		Run: func(ctx context.Context, a delivery.StepAsk) (any, error) {
 			if err := m.Prepare(ctx); err != nil {
 				return nil, err