review: verdigris patina, honest Lane figures, tests that can fail (#102)

Two-axis review found the shipped colour and three weak tests.

- --patina was a warm gold at the same hue family as --brass; the spec
  asked for a cool blue-green so an unhealthy Lane is unmistakably not
  ember. Now verdigris in both branches, with docs/design-system.md
  stating why the far side of the wheel is the point.
- A Lane pass that returns before computing its figures carries the
  previous pass's due count and gap forward instead of recording zeroes,
  and Checked rides beside Due so a stopped Lane is distinguishable from
  a quiet one.
- TestAdminPageWithoutAPollerSaysSo now separates the two causes it
  conflated, TestOwnerClearsReaderMarks seeds real counters so the
  clearing assertion can fail, and TestLaneStatus asserts Checked and
  the carry-forward.
- backend/AGENTS.md records the one deliberate owner comparison outside
  requireOwner and the carry-forward rule.

backend/internal/latest/status.go

@@ -3,14 +3,19 @@ package latest
 import "time"
 
 // LaneState is the administrative page's view of one Poll Lane (issue #102):
-// what the Lane's last completed pass saw. Due and Gap are filled in as the
-// pass computes them, so a pass that returned before reaching a figure (Lane
-// in refusal backoff, no fetcher) records a zero in its place.
+// what the Lane's last pass saw. Due, Gap and Checked are filled in as the
+// pass computes them; a pass that returned before reaching a figure (refusal
+// backoff, sidecar down) carries the previous pass's figures forward rather
+// than overwriting them with zeroes the page would state as fact.
 type LaneState struct {
-	Site     string
-	Due      int
-	LastRun  time.Time
-	Gap      time.Duration
+	Site    string
+	Due     int
+	LastRun time.Time
+	Gap     time.Duration
+	// Checked is how many Series this pass actually read. A Lane with Series
+	// due and nothing checked has stopped working; one with nothing due is
+	// merely quiet, and the page must not draw the two the same (story 13).
+	Checked  int
 	Clamped  bool
 	Refusing bool
 	Browser  bool
@@ -53,11 +58,17 @@ func (p *Poller) LaneStatus() Status {
 
 // recordLaneState stores one Lane's last pass for LaneStatus. Called deferred
 // from runLanePass so every return path records, even a pass that refused.
+// A pass that never reached the pace (Gap zero) keeps the last pass's figures:
+// the Lane's due count and gap did not become zero because this pass declined
+// to look, and the row's own marks say why it declined.
 func (p *Poller) recordLaneState(st LaneState) {
 	p.mu.Lock()
 	defer p.mu.Unlock()
 	if p.laneStates == nil {
 		p.laneStates = make(map[string]LaneState)
 	}
+	if prev, ok := p.laneStates[st.Site]; ok && st.Gap == 0 {
+		st.Due, st.Gap, st.Clamped, st.Checked = prev.Due, prev.Gap, prev.Clamped, prev.Checked
+	}
 	p.laneStates[st.Site] = st
 }