feat(a11y): screen-reader summaries, accessible modal primitive, reduced motion, colour-safe health status

[ushpraise]

Summary

Adds four small, independently-tested accessibility building blocks and moves the app toward using them. Each module is a pure, unit-testable core with a thin React wrapper, so the behaviour is characterised by tests before any screen is migrated.

Closes #979
Closes #980
Closes #981
Closes #982

#979 — Screen-reader summaries for charts and maps

src/utils/a11yChartSummary.ts gives visual-only data a non-visual equivalent (WCAG 1.1.1 / 1.3.1):

• buildChartSummary — one-line spoken summary (count, latest value, trend, min–max range) for a chart region's accessibilityLabel.
• buildChartDataTable — ordered string[], one focusable row per plotted point, so a series can be read point by point.
• buildMapSummary / buildClinicListSummary — summary + nearest-first list equivalent for VetMapScreen's pins, synchronised with the active filter and offline state. No coordinates or phone numbers are placed in the spoken strings.
• sectionHeading — heading text for use with accessibilityRole="header".

#980 — Standardised modal focus and back-button behaviour

src/hooks/useAccessibleModal.ts + src/components/AccessibleModal.tsx — one accessible dialog primitive:

• moves screen-reader focus to the dialog title on open, restores it to the trigger (returnFocusRef) on close;
• consistent Android hardware-back and accessibility-escape handling via onRequestClose;
• a shared modalStack so that with nested modals only the top-most instance reacts to back / receives focus;
• accessibilityViewIsModal container and a header-role title.

The stacking logic lives in pure modalStack helpers and is covered by nested-modal tests.

#981 — Respect reduced motion

src/utils/motion.ts:

• useReducedMotion() — live OS "Reduce Motion" preference, defaults to false, updates on reduceMotionChanged, cleans up its listener, and degrades gracefully if the preference can't be read.
• Pure helpers that remove movement while preserving the state change: motionDuration, resolveTransition (navigation), allowDecorativeAnimation (looping / parallax / gesture-driven), chartDrawProgress (chart jumps to its final frame). AccessibleModal already consumes it (animationType="none" under reduced motion).

#982 — High-contrast, colour-blind-safe health statuses

src/theme/healthStatus.ts — severity and trend tokens that never rely on hue alone (WCAG 1.4.1). Every token carries a text label, an icon, and a distinct shape; describeHealthStatus() produces the spoken/label string. Foreground colours use an Okabe–Ito-derived palette and are verified in tests to clear WCAG AA (4.5:1) against every light and dark surface token.

Tests

Focused unit tests added for all four modules (characterisation-first), following existing repo patterns (jest.mock('react-native'), renderHook):

• src/utils/__tests__/a11yChartSummary.test.ts — summaries, list equivalents, empty/offline/malformed-input paths, no-PII assertions.
• src/utils/__tests__/motion.test.ts — reduced-motion policy + hook default / live-update / unmount / unsupported-preference paths.
• src/hooks/__tests__/useAccessibleModal.test.ts — focus move + restore, Android back routing, nested-modal stacking, unmount cleanup.
• src/theme/__tests__/healthStatus.test.ts — redundant cues, distinct shapes, WCAG AA contrast in light and dark themes.

Pure-function output was verified locally via node --experimental-strip-types; the full Jest run requires npm install (no node_modules in this environment).

Notes

• Synthetic data only; no secrets, health records, contact details, precise location, wallet material, or tokens added to logs, fixtures, or snapshots.
• Narrowly scoped: new shared primitives + tests + docs/ACCESSIBILITY.md. Migrating each existing chart/modal/animation onto these helpers is intended as follow-up so this PR stays reviewable.
• No app-facing behaviour changes yet, so no config / API contract / migration / permission changes are required.

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