Add pinned WPT source preparation

[andrewiggins]

Summary

This PR is 1 of 3 (#631, #632) that adds a Web Platform Test runner to this repo, so we can validate our polyfills spec compliance against that test suite.

This PR focuses on just downloading an archive of the WPT repo we can use to read test files. Future PRs setup the runner to execute them against our DOM polyfills. Since we are on NodeJS 24, all scripts are written in TypeScript and executed directly in Node.

Specifically, this PR:

• add a private workspace package for preparing upstream Web Platform Test sources
• pin the WPT revision and archive SHA-256 checksum
• validate archive paths and links before extraction
• publish revision-addressed caches atomically
• serialize concurrent preparation with abandoned-lock recovery

Executable outcome

Run pnpm wpt:prepare to prepare or reuse the verified checkout. Set WPT_ROOT to use an existing checkout or WPT_CACHE_DIR to override the cache.

This PR does not add Vite, browser serving, WPT execution, capability claims, or CI.

Validation

• pnpm run lint
• pnpm run type-check
• pnpm exec vitest run
• pnpm wpt:prepare

Changeset

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