Add Web Platform Test runner infrastructure

[andrewiggins]

Summary

• add a private @remote-dom/wpt-runner workspace package for running selected upstream testharness.js HTML files unchanged against @remote-dom/polyfill
• pin and checksum the upstream WPT archive, with validated extraction and revision-addressed caches that are safe to share across concurrent worktrees
• add a canonical TSV capability inventory that distinguishes supported cases from documented deferred cases and rejects missing or unlisted results
• add Playwright CLI and debug-page entry points plus a dedicated Ubuntu 22.04 CI job with cached WPT sources

Architecture

The browser control page parses the selected WPT HTML and preserves top-level markup and script order. It sends those operations to a fresh module worker, where the workspace polyfill installs a new Window and executes the real upstream testharness.js. Results return as structured data for capability enforcement.

This infrastructure tests the worker-side DOM implementation directly; it intentionally does not construct a Remote DOM host receiver or claim transport and rendering behavior. The initial capability table contains only the runner smoke fixture. Follow-up PRs can add upstream DOM, HTML, and SVG files one capability at a time.

Validation

• pnpm run lint
• pnpm run type-check
• pnpm exec vitest run — 192 tests passed
• pnpm run test:wpt
• verified a cold archive download/extraction and the revision-root completion-marker layout
• verified WPT_ROOT=/path/to/wpt reuses a local checkout without downloading