docs: say that the board has two shapes now

F1 described "one lane per game" as a fact about the timeline, which is
now false half the time, and the prefs key space did not list the pref
that decides it. Both say why the merged board sorts with
endingSoonestFirst rather than a bare end date, since that is the part a
later change is most likely to "simplify".

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -126,6 +126,7 @@ src/
       sort.ts           deadline order, or what you're partway through
       lens.ts           who sees which rows — focus, outstanding, next-to-expire
       zoom.ts           the timeline's scale ladder; pure
+      lanes.ts          how the timeline stacks — lanes, or one deadline queue; pure
       useAppUpdate.ts   is a newer build waiting, and taking it   (F14)
 serve.ts                static server + /api/health              ✓ built
 scripts/