site: clarify mobile architecture scrolling

[kabudu]

Summary

• add an explicit mobile swipe/arrow-key cue for the wide architecture illustration
• make the scroll region keyboard-focusable and bind it to its instruction and claim-boundary caption
• validate the affordance and focus treatment as part of the static-site gate

Acceptance evidence

• python3 scripts/validate-pages-site.py — passed
• python3 scripts/build-pages-site.py — passed
• live-sized local inspection at 390×844 — no page overflow or broken assets; cue, scrollbar, and semantics present
• ./scripts/ci-local.sh — passed before commit

Finding addressed

The first live mobile inspection showed the intentionally wide diagram clipped at its initial position without an explicit horizontal-scroll affordance. The full diagram remained available, but discoverability was insufficient for launch quality.

[kabudu]

Reviewed head 3851d39edf2e5b3d7b0def725e9e5d93ee12a365 passed:

• full local ./scripts/ci-local.sh before commit
• hosted portable CI run 31324576508
• full local ./scripts/ci-local.sh again at the exact reviewed head
• complete remote-diff inspection with no review threads
• local 390×844 visual inspection with no page overflow or broken assets

Proceeding with the privacy-safe local squash identity.

[kabudu]

Merged by privacy-safe local squash commit on master after complete remote-diff review and successful local and hosted CI.