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/poller_test.go

@@ -1640,6 +1640,27 @@ func TestLaneStatus(t *testing.T) {
 	if !st.BrowserConfigured || !st.BrowserReachable {
 		t.Fatalf("browser after round = configured=%v reachable=%v, want true/true (sidecar never lost)", st.BrowserConfigured, st.BrowserReachable)
 	}
+	if asura.Checked != 1 {
+		t.Fatalf("asura Checked = %d, want the one Series it read", asura.Checked)
+	}
+
+	// A pass that declines to look (kagane is now in backoff) must not restate
+	// the figures it never gathered as zeroes: the last real pass's due count
+	// and pace stand until a pass replaces them.
+	before := kagane
+	if before.Due == 0 || before.Gap == 0 {
+		t.Fatalf("kagane after its refusing pass = %+v, want the figures that pass gathered", before)
+	}
+	p.runOnce(context.Background())
+	for _, lane := range p.LaneStatus().Lanes {
+		if lane.Site != "kagane" {
+			continue
+		}
+		if lane.Due != before.Due || lane.Gap != before.Gap {
+			t.Fatalf("kagane after a skipped pass = due %d gap %s, want the previous pass's %d / %s",
+				lane.Due, lane.Gap, before.Due, before.Gap)
+		}
+	}
 
 	// A lost sidecar reads as unreachable for the same window the Lanes skip.
 	p.setBrowserDown(now)