fix(ios): do not assign the 'spring' curve as a timing function

[farfromrefug]

Summary

Any view animated with curve: 'spring' crashes the app on iOS.

_createBasicAnimation uses curve to pick CASpringAnimation, then falls through to basicAnimation.timingFunction = curve and assigns the string 'spring' where CoreAnimation expects a CAMediaTimingFunction. The assignment is silent. The abort comes later, when the animation is committed:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException',
reason: '-[NSTaggedPointerString _getPoints:]: unrecognized selector sent to instance'

-[CASpringAnimation _copyRenderAnimationForLayer:]
CA::Layer::commit_animations(...)
_UIApplicationFlushCATransaction

Because it throws at commit time rather than at the call site, the crash lands on whatever frame comes next — which makes it look unrelated to the animation that caused it.

Every other curve value is a real CAMediaTimingFunction, so only 'spring' is affected. This is fork-specific: the curve === 'spring' ? CASpringAnimation : CABasicAnimation branch does not exist upstream.

Testing

Reproduced in an app that animates a slider thumb with curve: 'spring': consistent SIGABRT at the next CoreAnimation commit, matching the trace above. With this change the spring animation runs and the crash is gone.

CASpringAnimation still gets its damping, duration, fromValue and toValue as before — only the bogus timingFunction assignment is skipped.