Song switching and deletion aren't blocked during playback

[ironprogrammer]

Part-level editing is gated on playing (PartBlock.jsx disables the name, measures, cue, delete, duplicate and drag handle), but SongSwitcher never receives playing, so mid-playback you can still:

• switch to a different song, while the engine is running against the old one's parts/tempo
• delete a song, including the one currently playing — which also pops the undo toast over the transport

Both should be unavailable while playing is true, consistent with the rest of the song meta.