wld.promoteAndSwitch

Promote the source-file edits made during play (the play-shadow set — what accepting the "edits" group promotes), then switch the engine to the requested mode, in one call. Replaces the changes() / acceptChanges("edits") / wld.edit()-or-wld.play() sequence for the edit-and-retest loop. Entity changes stay in the sceneAuthoring review flow: a flip to edit with unaccepted entity changes stops at the leave-play safeguard.