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.

docs/design-system.md

@@ -73,7 +73,7 @@ Defined once in `backend/internal/web/static/style.css` `:root`, mirrored in the
 | `--moss` | `#7fae86` | `#3d6c46` | finished accent |
 | `--clay` | `#b5906f` | `#7c5533` | set-chapter accent |
 | `--trash` | `#977671` | `#8c6558` | remove, at rest — icons need 3:1, not 4.5:1 |
-| `--patina` | `#b08a4a` | `#7a5a1e` | admin page only — a Poll Lane needing attention, a Reader whose reports are blocked |
+| `--patina` | `#5fb3a6` | `#1f6f66` | admin page only — a Poll Lane needing attention, a Reader whose reports are blocked |
 | `--play-hot-line` | `#3a1d18` | `#f0cfc6` | desktop cell border, play when `.is-new` |
 | `--fav-line` | `#332b14` | `#e3d3a4` | desktop cell border, favourite when on |
 | `--asura` | `#7d93a5` | `#4f6b80` | site tag |
@@ -85,8 +85,9 @@ Defined once in `backend/internal/web/static/style.css` `:root`, mirrored in the
 `--slate`/`--moss`/`--clay`/`--brass`/`--patina` are held at the same weight
 deliberately: one accent per meaning, so a press says which lane it belongs to,
 with none of them competing with ember. `--patina` is the admin page's only
-colour — system health is neither a new chapter nor destruction, so it borrows
-neither `--ember` nor `--danger`.
+colour — a cool verdigris, the far side of the wheel from ember's crimson and
+clear of the archive blue: system health is neither a new chapter nor
+destruction, so it borrows neither `--ember` nor `--danger`.
 Dark is the default (`color-scheme: dark light`);
 light is a `@media (prefers-color-scheme: light)` override of the same names.
 **Any new colour must be added in both branches** — light is not a filter over