Enforce Web Platform Test capabilities in CI

[andrewiggins]

Summary

This PR is 3 of 3 (#630, #631) that adds a Web Platform Test runner to this repo, so we can validate our polyfills spec compliance against that test suite.

This PR adds a CLI to run the webpage from the previous PR in playwright. It also adds a way to specify which tests in WPT are expected to pass/fail, and why (if failure). So overtime we can document behaviors we support, ones we don't support yet, and which ones we explicitly don't want to support.

Specifically, this PR:

• add a canonical TSV inventory of supported and deferred WPT cases
• reject supported failures, missing results, unlisted results, duplicates, harness errors, worker errors, and timeouts
• report passing deferred cases as promotion candidates
• add a Playwright CLI that reuses one control page while creating a fresh worker per file
• add a dedicated cached WPT CI job

Stacked dependency

This PR is stacked on wpt-runner-interactive, which provides secure serving and isolated worker execution.

Executable outcome

Run pnpm test:wpt to execute every classified path and enforce the complete capability table. Use explicit paths for exploratory execution and --capabilities to enforce selected classified paths.

Validation

• pnpm run lint
• pnpm run type-check
• pnpm exec vitest run
• pnpm run test:wpt
• classified three-case smoke execution

Changeset

An empty changeset records that this private test infrastructure releases no public package.