test(stories): StoryCard shield a11y tests

### Area / Component

Stories

### Test Cases

- Verify hidden content, reveal behavior, and ARIA state.
- Guard against regressions in safety affordances and accessibility.
- Content not present before reveal; after click, visible; ARIA reflects expanded state.

Notes / Links
- Files: `src/__tests__/StoryCardShield.test.tsx`

### Target Release

v0.3.1