fix: an asleep browser Lane is not a stalled one on the admin page

laneRow.Stalled was Due > 0 && Checked == 0, which is exactly the shape
of the on-demand wake gate declining to start Chrome (ADR-0005): under
five due Series and none overdue by 15m, a browser Lane reads nothing on
purpose. kagane and novelfull each have one bookmarked Series, so that is
their state for most of every cycle and the page marked both "not
checking" — spending the one mark that means "go look" on the commonest
healthy state.

LaneState.Asleep is set at the wake gate, excluded from Stalled, kept out
of Attention, and rendered as "browser asleep". Verified in prod: those
Lanes wake on the age rule 15m past due (kagane and novelfull both polled
at 14:55Z for a 14:40Z due moment, novelfull picking up Chapter 4458).

backend/AGENTS.md

@@ -266,3 +266,9 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   than recording zeroes; a Lane that has never reached a pace renders no gap at
   all. `Checked` next to `Due` is what separates a stopped Lane from a quiet
   one, so neither figure may be dropped from the row.
+  Due-without-Checked is *not* by itself a stall: a browser Lane under both
+  wake thresholds sets `LaneState.Asleep` at the on-demand gate and renders
+  "browser asleep" instead of "not checking", and never counts toward
+  `Attention`. That is the commonest healthy state for kagane, comix and
+  novelfull — one due Series, nothing checked — so spending the stall mark on
+  it would train the owner to ignore the mark that matters.