fix: an asleep browser Lane is not a stalled one on the admin page

laneRow.Stalled was Due > 0 && Checked == 0, which is exactly the shape
of the on-demand wake gate declining to start Chrome (ADR-0005): under
five due Series and none overdue by 15m, a browser Lane reads nothing on
purpose. kagane and novelfull each have one bookmarked Series, so that is
their state for most of every cycle and the page marked both "not
checking" — spending the one mark that means "go look" on the commonest
healthy state.

LaneState.Asleep is set at the wake gate, excluded from Stalled, kept out
of Attention, and rendered as "browser asleep". Verified in prod: those
Lanes wake on the age rule 15m past due (kagane and novelfull both polled
at 14:55Z for a 14:40Z due moment, novelfull picking up Chapter 4458).

backend/internal/latest/poller_test.go

@@ -1388,6 +1388,11 @@ func TestBrowserLaneWakeThresholds(t *testing.T) {
 	if got := browser.callCount(); got != 0 {
 		t.Fatalf("browser fetches with 3 freshly-due series = %d, want 0 (Chrome stays asleep)", got)
 	}
+	// The owner's page reads this state off the snapshot, and Due-without-
+	// Checked has to be distinguishable there from a Lane that has stopped.
+	if lane := laneByName(t, p, "kagane"); !lane.Asleep || lane.Due != 3 || lane.Checked != 0 {
+		t.Fatalf("asleep kagane lane = %+v, want Asleep with 3 due and 0 checked", lane)
+	}
 	// 5 due crosses the count threshold.
 	for i := 3; i < 5; i++ {
 		seed(i)
@@ -1396,6 +1401,9 @@ func TestBrowserLaneWakeThresholds(t *testing.T) {
 	if got := browser.callCount(); got != 5 {
 		t.Fatalf("browser fetches with 5 due series = %d, want 5", got)
 	}
+	if lane := laneByName(t, p, "kagane"); lane.Asleep {
+		t.Fatalf("woken kagane lane still reports Asleep: %+v", lane)
+	}
 	// A single long-neglected series wakes the browser by age alone.
 	seedForCheck(t, s, "kagane:ancient", "https://kagane.to/series/ancient", 0)
 	p.runOnce(context.Background())
@@ -1404,6 +1412,19 @@ func TestBrowserLaneWakeThresholds(t *testing.T) {
 	}
 }
 
+// laneByName pulls one Lane out of the poller's snapshot, failing rather than
+// returning a zero LaneState a caller would assert against by accident.
+func laneByName(t *testing.T, p *Poller, site string) LaneState {
+	t.Helper()
+	for _, lane := range p.LaneStatus().Lanes {
+		if lane.Site == site {
+			return lane
+		}
+	}
+	t.Fatalf("no %q lane in the snapshot", site)
+	return LaneState{}
+}
+
 // When one browser Lane loses the sidecar, the round's remaining browser
 // Lanes are skipped: every fetch would fail anyway, and their Series must not
 // burn their stamps on a dead Chrome (issue #100).