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

[Copilot]

Pull request overview

This PR fixes a race where wrapping an empty keep-alive queue in UniformSourceIterator could cache incorrect initial metadata (mono) and apply the wrong channel conversion to the first samples appended shortly after, causing a permanent 1-sample channel shift.

Changes:

• Make UniformSourceIterator lazily bootstrap its converters on the first next() call (instead of during new()), so it observes up-to-date source metadata.
• Adjust queue span-length reporting to use self.channels() (which can reflect the next queued source) rather than the current exhausted source’s channel count.
• Add a regression test ensuring the first appended stereo samples are emitted with correct L/R ordering when added to a mixer after connecting an initially-empty keep-alive queue.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/source/uniform.rs	Defers converter initialization until first sample pull using a pending source, preventing early metadata capture from placeholder sources.
src/queue.rs	Ensures span-length fallback aligns with the queue’s effective channel count and adds a regression test for correct first-append channel behavior.

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