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/web/static/style.css

@@ -87,10 +87,11 @@
   --moss: #7fae86;         /* finished */
   --clay: #b5906f;         /* set chapter */
   --trash: #977671;        /* remove, resting — icons need 3:1, not 4.5:1 */
-  /* A Lane needing attention: the admin page's only accent, held at the same
-     muted weight as --brass so it never competes with ember. Neither ember
-     (new chapter) nor danger (destruction) may say "system unhealthy". */
-  --patina: #b08a4a;
+  /* A Lane needing attention: the admin page's only accent. Verdigris — cool,
+     the far side of the wheel from ember's crimson, and clear of the archive
+     blue. Neither ember (new chapter) nor danger (destruction) may say
+     "system unhealthy". */
+  --patina: #5fb3a6;
 
   /* Desktop cell borders for the two coloured action states. */
   --play-hot-line: #3a1d18;
@@ -150,7 +151,7 @@
     --moss: #3d6c46;
     --clay: #7c5533;
     --trash: #8c6558;
-    --patina: #7a5a1e;
+    --patina: #1f6f66;
 
     --play-hot-line: #f0cfc6;
     --fav-line: #e3d3a4;