41: Accessibility baseline pass (keyboard, ARIA, contrast, reduced motion)

[Saber5656]

Derived from docs/issues/41-accessibility-baseline.md — the canonical source of truth. Execution order & dependencies: docs/ISSUE_PLAN.md (PR #1). If this issue and the doc disagree, the doc wins.

Summary

Execute the DESIGN §11.2 checklist across all shipped components: keyboard operability, focus
management, ARIA labeling, contrast tokens, reduced-motion audit, and a year-change screen-reader
announcer — with automated assertions where practical.

Context

A map app tends to be pointer-only by accident. §11.2 sets a pragmatic baseline (not full WCAG
conformance) appropriate for v1 OSS.

Scope

• Audit + fix per component (08–42 set): tab order (controls-top → map → dock → sheet when open),
  visible :focus-visible ring token, Esc closes popovers/sheets, focus trap in sheets (22 host —
  verify + fix), focus return to opener on close.
• src/ui/a11y/announcer.ts: visually-hidden aria-live="polite" region; announce year changes
  throttled to settle (i18n slider.valuetext reuse) and layer switches (announce.layerChanged).
• Contrast: verify chrome tokens (08) ≥ 4.5:1 text / 3:1 UI on both light/dark; adjust token
  values; document measured ratios in PR (tooling: any contrast checker; record numbers).
• Map-over-imagery legibility: badges/banners get scrim backgrounds (token
  --color-scrim) — verify over bright aerial fixture.
• Reduced motion: grep all animations (crossfade 18, flyTo 13/21, sheet transitions 22, toast 40,
  onboarding 42) honor prefers-reduced-motion — add missing guards.
• Touch targets: assert ≥ 44px on LocateButton, PoiToggle, MenuButton, slider thumb hit area,
  sheet close, toast action (e2e bounding-box checks).
• Zoom/scale: page functional at 200% browser zoom (no clipped controls).

Detailed Requirements

1. Every fix lands in the owning component's file; this issue may touch many files but only for
   a11y deltas (no refactors).
2. Add e2e tests/e2e/a11y.spec.ts: keyboard-only journey (tab to slider → change year → tab to
   locate → activate → open layers sheet → Esc) with focus assertions; touch-target size
   assertions; aria-live region updated on year change.
3. Lighthouse a11y ≥ 90 in 46's CI run (this issue makes it true; 46 enforces).

Acceptance Criteria

• Keyboard-only journey e2e passes with zero pointer events.
• Announcer emits on year settle (not per drag frame) — asserted via DOM text.
• Contrast table in PR (token → ratio → pass) covering text, badges, banner, toast, buttons.
• Reduced-motion e2e (emulate media) shows instant layer swap + jumpTo.

Validation

tests/e2e/a11y.spec.ts; manual VoiceOver (iOS) smoke noted in PR (best-effort, non-blocking).

Dependencies

Components through 40 (audits them). Blocks nothing; 46 measures the result.

Non-goals

Full WCAG 2.2 conformance claim, screen-reader map exploration (canvas map limits — out of scope),
high-contrast theme (v2).

Design References

DESIGN §11.2, §6.3 (slider ARIA), §8.