fix(cli): resolve race condition in locale processing

[ceolinwill]

Summary

@vrcprl #1662 didn't fully fix #1661 because there's a race condition in _utils.ts where pullInput and pullOutput are stored as single values that get overwritten on each pull() call

Changes

• Changed state storage from single values to per-locale using Map

Testing

Business logic tests added:

• Check the Language header is correct on concurrent processing
• All tests pass locally

I also made sure to test locally with npm link this time. Sorry I didn't do it last time.

Visuals

Required for UI/UX changes:

• Before/after screenshots attached
• Video demo for interactions (< 30s)

Checklist

• Changeset added (if version bump needed)
• Tests cover business logic (not just happy path)
• No breaking changes (or documented below)

Closes #1661