A build seat's capacity follows its node's build CPU, and a running ask says its phases

Since the build CPU cap (issue 1806) shanks' mesh-build-go kept capacity 3 inside a 12-core slice:
three full checks got about 4 cores each and ran past the 30-minute step bound (issue 1890).
The puller now judges by min(configured, floor(slice quota cores / cores-per-step)), the quota
read live from its cgroup, says it on change and writes capacity.<node> to the pauses state.
A run's phases (fetch and merge, digest, tests) are logged with the ask id and written to its
running step key; Asker.Wait hands every step value to a WithStepSeen hook.

go/buildstep/pull_test.go

@@ -104,20 +104,38 @@ func TestPullTakesDeliveryBeforeCheck(t *testing.T) {
 }
 
 // 1887/A2a, 1887/AC-pull: given capacity 2 and three asks waiting, two run and the third waits, and it starts when
-// one ends, with no timer.
+// one ends, with no timer. Issue 1806's follow-up: the 2 is the CPU's, a configured 3 bounded by a 12-core slice at 6
+// cores a step, said once in capacity.<node>; and issue 1890: a run's phase is written to its step key and logged with
+// its ask id, and its end keeps the last phase.
 func TestPullHoldsAtMostItsCapacity(t *testing.T) {
 	src, st := NewFakeAsks(), newSteps()
+	asks := []delivery.StepAsk{}
 	for i := 0; i < 3; i++ {
-		_ = src.PublishAsk(testAsk(delivery.AskDelivery))
+		a := testAsk(delivery.AskDelivery)
+		asks = append(asks, a)
+		_ = src.PublishAsk(a)
 	}
 	release := make(chan struct{})
 	var mu sync.Mutex
 	started := 0
-	p := &Puller{Source: src, Language: delivery.AskGo, Node: "shanks", Put: st.put, Capacity: func() int { return 2 },
+	var said []string
+	capacities := map[string][]byte{}
+	quota := 1200
+	p := &Puller{Source: src, Language: delivery.AskGo, Node: "shanks", Put: st.put, Capacity: func() int { return 3 },
+		CoresPerStep: func() float64 { return 6 },
+		Quota:        func() (int, string) { mu.Lock(); defer mu.Unlock(); return quota, "a test's slice" },
+		PausePut: func(key string, v any) (uint64, error) {
+			mu.Lock()
+			capacities[key] = v.(json.RawMessage)
+			mu.Unlock()
+			return 1, nil
+		},
+		Logf:     func(f string, a ...any) { mu.Lock(); said = append(said, fmt.Sprintf(f, a...)); mu.Unlock() },
 		PullWait: 10 * time.Millisecond, Run: func(ctx context.Context, a delivery.StepAsk) (any, error) {
 			mu.Lock()
 			started++
 			mu.Unlock()
+			Phase(ctx, "unit: tests of 3 packages")
 			<-release
 			return nil, nil
 		}}
@@ -129,12 +147,49 @@ func TestPullHoldsAtMostItsCapacity(t *testing.T) {
 	if p.InHand() != 2 || src.Waiting(delivery.AskDelivery) != 1 {
 		t.Fatalf("in hand %d, waiting %d; want 2 and 1", p.InHand(), src.Waiting(delivery.AskDelivery))
 	}
+	c := p.CapacityDetail()
+	if c.Effective != 2 || c.Configured != 3 || c.ByCPU != 2 || c.QuotaCores != 12 || !strings.Contains(c.Why, "bounded by the CPU") {
+		t.Fatalf("the capacity judged by = %+v, want 2 of a configured 3 bounded by 12 cores / 6", c)
+	}
+	mu.Lock()
+	raw := capacities["capacity.shanks"]
+	capLines := 0
+	for _, l := range said {
+		if strings.Contains(l, "build capacity of shanks is 2, bounded by the CPU") {
+			capLines++
+		}
+	}
+	mu.Unlock()
+	if got, err := delivery.DecodeCapacity(raw); err != nil || got.Effective != 2 || capLines != 1 {
+		t.Fatalf("capacity.shanks = %s (%v), said %d times; want effective 2, said once", raw, err, capLines)
+	}
+	waitFor(t, "a phase on the step key", func() bool {
+		key, _ := delivery.AskStepKey("shanks", p.Running()[0].ID)
+		s, _ := st.get(key)
+		return s.State == delivery.StepRunning && s.Phase == "unit: tests of 3 packages" && !s.PhaseAt.IsZero()
+	})
 	release <- struct{}{}
 	waitFor(t, "the third taken", func() bool { return src.Waiting(delivery.AskDelivery) == 0 })
 	// Taken from the source comes a moment before its run starts: wait for the start, not only the take.
 	waitFor(t, "the third started", func() bool { mu.Lock(); defer mu.Unlock(); return started == 3 })
 	close(release)
 	waitFor(t, "all acked", func() bool { a, _, _ := src.Fates(); return len(a) == 3 })
+	for _, a := range asks {
+		key, _ := delivery.AskStepKey("shanks", a.ID)
+		if s, _ := st.get(key); s.State != delivery.StepPassed || s.Phase != "unit: tests of 3 packages" {
+			t.Errorf("the end of %s = %+v, want passed in its last phase", a.ID, s)
+		}
+	}
+	mu.Lock()
+	logged := strings.Join(said, "\n")
+	quota = 600 // the cap lowered: one step at 6 cores, followed at the next judgement
+	mu.Unlock()
+	if !strings.Contains(logged, "the delivery ask "+asks[0].ID+" on shanks: unit: tests of 3 packages") {
+		t.Errorf("no journal line names the ask's phase: %s", logged)
+	}
+	if c := p.CapacityDetail(); c.Effective != 1 {
+		t.Errorf("after the quota fell to 6 cores, capacity %+v, want 1", c)
+	}
 }
 
 // An ask that is not one, or not this seat's, is dropped and runs nowhere. And issue 890/AC1: a check ask needing