test: add deterministic component interaction harness

[ddebowczyk]

Summary

• add React Testing Library and user-event with a small shared Layman provider harness
• cover empty, tab selection, close, maximize/restore, and floating resize interactions
• add accessible names for icon-only controls used by the tests

Validation

• npx vitest run tests/Layman.interaction.test.tsx --pool=threads --maxWorkers=1
• NODE_OPTIONS="--localstorage-file=/tmp/react-layman-vitest-localstorage" npm test (75 passed)
• npm run lint -- --max-warnings=0
• strict TypeScript check of the two new test files
• npm run build:lib

Dependency

This PR follows #29 for Node 26 portable localStorage test setup. On upstream main, plain npm test has the pre-existing persistence-suite failure; this PR does not duplicate that setup change.