refactor: make browser runtime optional

[shenjiecode]

Summary

• move playwright-core from runtime dependency to an optional peer
• lazy-load the browser runtime so core activation works without Playwright
• add Node/Pi compatibility matrix documentation and scheduled/manual CI
• verify ordinary installs do not automatically install Playwright

Verification

• npm run verify (48 tests)
• build smoke with and without Playwright
• local minimum Pi 0.80.2 verification
• optional-peer clean-install smoke
• npm run pack:dry-run
• npm audit --omit=dev --audit-level=high

This PR is the source change only. The package release will be versioned separately after merge.