Follow up: Confirm SegmentButton default size

[georgewrmarshall]

Overview

Follow-up from #1172 to confirm the intended default size for SegmentButton.

Problem

SegmentButton currently defaults to ButtonBaseSize.Sm, but review feedback on #1172 suggested ButtonBaseSize.Md because most segment buttons use the medium size.

This should be resolved explicitly so the implementation, docs, stories, and design guidance all agree on the default size.

Acceptance Criteria

• Confirm whether the default SegmentButton size should be Sm or Md.
• If the default should be Md, update the component default and add/update tests.
• If the default should remain Sm, document that decision in the README or related design guidance.
• Ensure Storybook default args reflect the intended default behavior.

References

• PR: feat: Added SelectButton, SegmentButton, and SegmentGroup to DSRN #1172
• Review comment from Amanda: suggested size = ButtonBaseSize.Md
• File: packages/design-system-react-native/src/components/SegmentButton/SegmentButton.tsx