[flags] Enable enableParallelTransitions (#37290)

Reverse #35709. Turn the flag on in the OSS/canary default and in every
hardcoded fork, including the native (RN) forks, and keep the test
renderers in sync. www stays GK-driven, so `www.js` and `www-dynamic.js`
are unchanged.

Leaving the flag in the repo for now; this just flips the default on.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>