fix: prevent channel shift when appending to an empty keep-alive queue

[roderickvd]

When Player::connect_new calls mixer.add(queue) with an empty queue, UniformSourceIterator::bootstrap() captures channels=1 from the Empty placeholder and creates a Take(1) with a ChannelCountConverter(from=1). If a stereo source is then appended before the audio thread calls next(), that first sample passes through the wrong converter: L is treated as mono and duplicated, permanently shifting the stream by one sample and shifting channels.

Supersedes #870
Fixes #871