test(e2e): cover the similar-schools section, compare hand-off and mobile widths

Two things here cannot be covered anywhere else. jsdom has no layout, so
scrollWidth and clientWidth are both 0 and the arrows' disabled state can only
be measured by a real engine. And the scroll position surviving a selection is
DOM state rather than React state, so only a real browser can prove the row
does not jump back when the footer re-renders.

MOBILE.md asks for a Playwright width check and records that it was not written
because Playwright was not in the project. It is — this suite — so the check
exists now, scoped to the page this feature touches.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -73,7 +73,12 @@ There is no SWR dependency. Leaflet maps are loaded through dynamic wrappers;
 Chart.js renders performance and comparison charts.
 
 `components/school/` contains detail sections, with section decisions and data
-preparation in `lib/schoolSections.ts`. `lib/types.ts` contains manually maintained
+preparation in `lib/schoolSections.ts`. The similar-schools section is selected
+in `backend/similar_schools.py` — hard filters that never relax (phase,
+provision, selectivity, gender) and soft preferences that do (religious
+character, then gender exactness) — and served on `/api/schools/{urn}`. Its
+rules are presentation logic, deliberately kept out of `marts.*` so they can be
+tuned by deploy rather than by pipeline run. `lib/types.ts` contains manually maintained
 API types. `payload-types.ts` and the Payload import map are generated artifacts.
 
 ## Publication and caching today