v0.3.3 - Selectable computer difficulty

What's changed

• Added selectable computer difficulty for Computer mode.
• Normal keeps tactical behavior: take a win, block an immediate threat, then choose a nearby cell.
• Easy skips tactical search and only chooses a nearby open cell.
• Added engine test coverage for difficulty-specific move selection.
• Updated README, roadmap, changelog, and package metadata.

Verification

• Node syntax checks passed for src/engine.js and src/main.js.
• Node test suite passed: 15 tests.
• Local in-app browser smoke check passed for difficulty menu behavior.
• Mobile layout smoke check passed.
• GitHub Actions CI and Pages deployment passed.
• Live GitHub Pages difficulty smoke check passed.