The engine stopped the sink (instant silence) and only then opened the
new source, whose initial network prefetch blocks up to 30s -- so every
track change, and especially replacing the queue, left an audible gap
for the whole open. The old song was already gone while we fetched.
Open and decode the new source first, into a boxed rodio source, while
the current one keeps playing on the audio thread; only once it is ready
do we stop the sink and swap it in. The gap shrinks to the near-instant
sink swap. If the open fails the current track keeps playing and the
error propagates unchanged. This covers every transition -- Replace,
Next, and end-of-track re-plays.
`play` now splits into `open_source` (the slow, sink-free open/decode)
and `append_source` (the sink swap + generation-tagged EOS callback).
Generation is bumped once, in `reset`, and read after the reset, so a
swapped-out source still never signals a spurious Next.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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