feat(babel): add parallel option to transform in worker threads

[NullVoxPopuli-ai-agent]

Note

I am an AI bot: Claude Code, with the Claude Opus 5.5 model. @NullVoxPopuli asked me to open this PR. @NullVoxPopuli, please review.

Adds a parallel option to @rolldown/plugin-babel. With it, the plugin runs Babel in worker threads. This is a port of the same option from @rollup/plugin-babel (rollup/plugins#1956).

babel({
  parallel: true, // or a worker count
  plugins: [['@babel/plugin-proposal-decorators', { version: '2023-11' }]],
})

Behavior

• true starts one worker per CPU core, with a maximum of 4. A number sets the worker count.
• The pool starts on the first transform. It stops in closeBundle (not in watch mode) or closeWatcher.
• Babel options must be structured-cloneable. If a plugin or preset is a function or an object, the plugin throws at startup and names the option, for example overrides[0].presets[0].
• The rolldown part of a Rolldown Babel preset stays in the main thread. So its filters and hooks can still be functions.
• Babel errors from a worker keep loc, so Rolldown still reports foo.js:1:13. workerpool drops nested error fields, so the worker adds a toJSON.

The other recent performance change in @rollup/plugin-babel (rollup/plugins#1954, hook filters) already exists in this plugin.

Benchmark

600 files with decorators, built from dist/, on an 8-core machine. Two rounds each, with the order mirrored:

parallel	build time
off	83 to 86 s
2	43 to 46 s
4	26 to 28 s
8	21 s

Notes for review

• New runtime dependency: workerpool@^10.0.3, the same package that the rollup plugin uses.
• Errors from loadOptionsAsync now also get the [BabelError] prefix, because they are now inside the same try block as the transform.
• The test build() helper now calls bundle.close(), so that worker pools stop.
• Not tested: the Vite dev server. The Rolldown builds and the existing Vite build tests pass.

🤖 Generated with Claude Code

[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	workerpool@​10.0.3

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[NullVoxPopuli]

seems good to me

[NullVoxPopuli]

idk if there is a better way to do this, but the purpose of doing this much work to see if a config is serializable is to provide a better error to consumers when part of their config is not serializable