fix(ui): improve segmented keyboard accessibility

[wyf027]

Summary

• Add radiogroup / radio semantics to Segmented so assistive tech can announce the grouped single-selection state.
• Add roving tabIndex and keyboard selection for Arrow keys, Home, and End while skipping disabled options.
• Add a top-level disabled prop and keep option-level disabled behavior.
• Add Segmented tests for ARIA state, click selection, keyboard navigation, disabled behavior, and first-enabled defaults.
• Document keyboard behavior plus disabled and SegmentedOption.disabled.

Gap Notes

• Ant Design Segmented documents grouping radio inputs by name so browsers preserve group behavior, including left/right arrow selection: https://ant.design/components/segmented/
• Element Plus Segmented supports disabled state at both component and option levels: https://element-plus.org/en-US/component/segmented
• WAI-ARIA / MDN radiogroup guidance expects a named group, checked radio state, and Arrow/Home/End style keyboard movement for radio groups: https://www.w3.org/WAI/ARIA/apg/patterns/radio/ and https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Reference/Roles/radiogroup_role

Validation

• npm ci
• npx prettier --write packages/ui/src/components/form/controls/Segmented/Segmented.tsx packages/ui/src/components/form/controls/Segmented/Segmented.test.tsx docs/components/segmented.md
• npm run lint:ui
• npm run typecheck:ui
• npm run test:ui (24 files, 75 tests)
• npm run build:ui
• npm run build

Notes

• npm ci still reports existing Node 23 engine warnings and existing npm audit findings (1 low, 4 moderate, 2 high).
• npm run build still reports existing Browserslist and VitePress chunk-size warnings.

[ecc-tools]

ECC bundle files are already tracked in this repository. Skipping generation of another bundle PR.