v0.3.7 - Accessibility regression checks

What's changed

• Added CI-backed accessibility regression checks for visible board focus styling.
• Added source checks for the polite result live region.
• Added source checks that the latest played stone remains exposed as the current step.
• Updated README, roadmap, accessibility notes, changelog, and package metadata.

Verification

• Node syntax checks passed for src/engine.js and src/main.js.
• Node test suite passed: 21 tests.
• git diff --check passed.
• GitHub Actions CI and Pages deployment passed.