Fix live HLS downloader not stopping on first Ctrl+C

[Copilot]

When downloading live HLS streams, the first Ctrl+C logged a stop message but did not actually stop the downloader if it was blocked waiting for the next segment batch. Users had to press Ctrl+C again to force exit.

• Downloader stop behavior

  • Updated ParallelDownloader::download to race stream polling against the stop signal.
  • Replaced post-iteration termination checking with immediate interruption via tokio::select!, so stop is honored even while waiting on stream.next().

• Regression coverage for live-wait scenario

  • Added a focused test that simulates a live source with delayed next batch emission.
  • Verifies downloader exits promptly after stop signal instead of waiting for the delayed batch.

• Test clarity cleanup

  • Removed unnecessary boxing in the test stream construction.
  • Replaced inline delay literal with a named constant for intent readability.

while let Some(segments) = tokio::select! {
    _ = &mut self.stop_signal => None,
    segments = stream.next() => segments,
} {
    // process segment batch
}