feat(feed): support feed role article focus management

[AdamMagued]

Description

Resolves #13.

This PR implements W3C WAI-ARIA feed role focus management according to the ARIA 1.2/1.3 specification:

• When the virtual cursor transitions into an article belonging to a feed (role="feed") or moves between different articles within a feed, user agent focus (.focus()) is set on the containing article.
• If the reading cursor lands directly on a focusable element inside the article, focus is set on that element in lieu of setting focus on the containing article.
• Moving within the same article does not repeatedly re-trigger focus unless changing to a focusable element.
• Non-feed articles do not trigger feed-specific focus shifts.
• Internal focus shifts are tracked to avoid disrupting reading cursor state.

Verification

• Added unit tests for focusable selector detection (test/isFocusable.test.ts).
• Added comprehensive integration tests (test/int/feedRole.int.test.ts) covering all specification focus behaviors.
• Ran full CI suite (yarn ci), passing with 100% test pass rate (59 suites, 349 tests), clean linting, and full typechecks.