test(hooks): finalize mutation test improvements and release acceptance

- Raise use-swipe-gesture mutation score to 66.13% (target 65%+)
- Maintain use-reduced-motion mutation score at 76.32% (target 50%+)
- Fix use-reduced-motion.ts ESLint set-state-in-effect warning
- Add UJ-10 deep searcher journey (category browse → article read → content discovery)
- Add 40 new test files (analytics, detail, sections, ui, lib components)
- Update test-strategy-plan.md to v2.0 (sync test count to 1591)
- Sync README.md with final release metrics

Quality gates: TS 0 errors, ESLint 0 errors, 121 suites / 1591 tests passed

src/lib/constants/design-system.ts

@@ -2,7 +2,7 @@
 // Color tokens reference global CSS variables for dark-mode support.
 // Layout / spacing / animation / easing remain as JS constants.
 
-export const DESIGN_SYSTEM = {
+export const DESIGN_SYSTEM = Object.freeze({
   animation: {
     duration: {
       fast: '0.2s',
@@ -136,6 +136,6 @@ export const DESIGN_SYSTEM = {
       description: 'text-xs mt-1 text-[var(--color-text-hint)]',
     },
   },
-};
+});
 
 export type DesignSystem = typeof DESIGN_SYSTEM;