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 against @remote-dom/polyfill without modifying the upstream files
• pin and checksum the downloaded WPT archive, validate it before extraction, and use 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, duplicate, or unlisted results
• isolate each test file in a fresh worker with a new polyfill Window, private result transport, and worker-specific network restrictions
• add Playwright CLI and debug-page entry points plus a dedicated CI job with cached WPT sources

Architecture

The browser control page parses each selected WPT file and turns supported top-level markup and classic scripts into generated source in their original order. The CLI reuses one control page across files, but each file runs in a fresh module worker with a new Window from @remote-dom/polyfill.

Runner results travel over a private MessagePort rather than the worker's public message channel. A worker-specific Content Security Policy blocks network connections and nested workers while allowing the same-origin module loading and generated-code execution required by the runner. Served WPT files use ETag revalidation to avoid repeatedly transferring shared resources such as testharness.js.

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
• pnpm run test:wpt
• verified a cold archive download and extraction, including the revision-root completion-marker layout
• verified concurrent preparation shares one validated archive and recovers abandoned locks
• verified WPT_ROOT=/path/to/wpt reuses a local checkout without downloading
• verified worker networking and nested workers are blocked
• verified the CLI reuses its control page while creating a fresh worker for each file

Changeset

An empty changeset is included because @remote-dom/wpt-runner is private test infrastructure and does not release a public package.

[andrewiggins]

Superseded by the three-PR replacement stack:

1. Source preparation: Add pinned WPT source preparation #630
2. Interactive runner: Add an interactive Web Platform Test runner #631
3. Capability enforcement and CI: Enforce Web Platform Test capabilities in CI #632

The replacement stack gives each PR a self-contained executable outcome. This PR is closed without merging.