Is handoff / continuity (iOS 8+) supported in iTunes to seamlessly switch playback between Mac and iOS?
Handoff is not supported in Music.app, however your use case (control media playback on your iPhone from a Mac) would be more along the lines of what the Apple Remote app does, just in reverse.

Tunes Remote allows you to control one device from another device or a Mac. I have not used the software, so YMMV.