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/templates/admin.html

@@ -27,7 +27,10 @@
     </form>
   </header>
 
-  {{template "lanes" .Lanes}}
+  {{/* The live region wraps the swapped block rather than being it: the
+       refresh replaces the section wholesale, and a region recreated on every
+       update is never announced. */}}
+  <div aria-live="polite">{{template "lanes" .Lanes}}</div>
 
   {{template "readers" .}}
 </div>