latest: report each Poll Lane's last pass to the owner's page (#102)

A Lane's pace, its refusal backoff and whether its Site needs the browser were
only ever visible in the log. The admin page (issue #102) has to state them, so
the Poller keeps one snapshot per Lane.

- LaneState/Status (status.go) is the page's view of a Lane: due, gap, clamped,
  refusing, browser.
- runOnce records a snapshot on every return path, including the pass that
  refuses, so a cooling Lane does not read as one that never ran.
- Refusing is derived at snapshot read time from the backoff, not stored: a
  Lane that cooled down between passes must report false without a new pass.
- LaneStatus reports only Sites that have completed a pass, so a fresh restart
  renders "no data yet" instead of confident zeroes.

backend/internal/latest/poller.go

@@ -57,6 +57,10 @@ type Poller struct {
 	mu            sync.Mutex
 	refuseUntil   map[string]time.Time
 	browserDownAt time.Time
+	// laneStates is the owner's page snapshot of each Lane's last pass
+	// (issue #102), keyed by Site. Guarded by mu; a Site appears only after
+	// its first pass, so a restart renders "no data yet" rather than zeroes.
+	laneStates map[string]LaneState
 	// coverWG tracks in-flight cover work. Covers heal in the background so a
 	// slow cover host cannot delay the next Series-page Poll; tests join it
 	// before asserting on cover fetches.
@@ -218,6 +222,10 @@ func (p *Poller) runOnce(ctx context.Context) {
 // production Lane's rate limit; the deterministic test entry runs back to back.
 func (p *Poller) runLanePass(ctx context.Context, name string, paced bool) time.Duration {
 	now := p.Now()
+	// Snapshot this pass for the owner's page (issue #102). Recorded on every
+	// return path, with the figures filled in where the pass computes them.
+	st := LaneState{Site: name, LastRun: now, Browser: isBrowserSite(name)}
+	defer func() { p.recordLaneState(st) }()
 	if until := p.refusalBackoff(name); now.Before(until) {
 		// Cooling down after a refusal: do not attempt this Site at all.
 		return until.Sub(now)
@@ -246,6 +254,7 @@ func (p *Poller) runLanePass(ctx context.Context, name string, paced bool) time.
 		log.Printf("latest poll %s: due query: %v", name, err)
 		return defaultGap
 	}
+	st.Due = len(due)
 	if s.Browser != nil && f == p.BrowserFetch && !browserWakeDue(due, now, s.Rest) {
 		// Below both thresholds Chrome stays asleep (ADR-0005 on-demand
 		// browser): waking it for a single Poll would cost a challenge solve
@@ -268,6 +277,7 @@ func (p *Poller) runLanePass(ctx context.Context, name string, paced bool) time.
 		return defaultGap
 	}
 	gap, clamped := effectiveGap(s, eligible)
+	st.Gap, st.Clamped = gap, clamped
 	if clamped {
 		log.Printf("latest poll %s: gap clamped to %s floor (eligible series=%d)", name, minGap, eligible)
 	}