feat: cancel superseded native-queue downloads

[RohithPariki]

Summary

Fixes #491.
This adds robust background task cancellation for native-queue downloads (Subsonic, Qobuz, YouTube) when the user skips tracks rapidly. It ensures we don't leak tokio tasks or needlessly waste network bandwidth/disk I/O on downloads that will be discarded.

Problem

In play_queued_subsonic, play_queued_qobuz, and play_queued_youtube, the tokio::spawn download task runs completely detached. If a user skips quickly, the queue slot is republished but the old task runs to completion. The stale result is only dropped when the download finally finishes and finish_decoded_fetch sees the fetch_id mismatch.

Solution

I implemented a Drop-driven cancellation pattern:

1. Created DownloadAbortHandle, a thin wrapper around tokio::task::AbortHandle that calls .abort() on Drop.
2. Added abort_handle: Option to DecodedQueuePlayback.
3. In the dispatch handlers, after spawning the background task, we take the App lock one more time. If the slot still carries the matching fetch_id, we inject the abort handle. If it's already advanced, we .abort() it immediately.
4. Because queue_now is overwritten on skips (or cleared on teardown), the old DecodedQueuePlayback is naturally dropped, bringing our DownloadAbortHandle with it and cleanly cancelling the background task instantaneously.

Testing

Added the regression test test_queue_skip_aborts_pending_download to src/infra/queue/dispatch.rs that explicitly verifies a skip clears the slot and correctly aborts the pending download task.

Related Issue

Fixes #491

Summary by CodeRabbit

• Bug Fixes
  • Improved cancellation of pending downloads for Subsonic, Qobuz, and YouTube playback.
  • Downloads now stop promptly when their queue slot is cleared or replaced, reducing unnecessary background activity.
  • Prevented orphaned background download tasks from continuing after playback items are removed or updated.

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• src/infra/queue/dispatch.rs

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📝 Walkthrough

Walkthrough

Adds abort-handle storage to decoded queue slots. Subsonic, Qobuz, and YouTube downloads now cancel when their slots are replaced or cleared. A regression test verifies cancellation of a pending download.

Changes

Queue download cancellation

Layer / File(s)	Summary
Abort handle contract src/infra/queue/mod.rs	Adds the feature-gated DownloadAbortHandle wrapper and the optional abort_handle field on DecodedQueuePlayback.
Download task registration and validation src/infra/queue/dispatch.rs	Stores abort handles for Subsonic, Qobuz, and YouTube downloads, aborts tasks when slots are superseded, initializes pending and local-file slots, and tests cancellation after slot clearing.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Low

Merge Risk: ⚪ Minimal · up to e9968

Superseded or cleared decoded queue downloads are cancelled through the stored abort handle, with no remaining actionable merge risk identified.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title uses the allowed feat: prefix, clearly describes cancellation of superseded native-queue downloads, and uses a concise imperative subject.
Linked Issues check	✅ Passed	The PR satisfies the coding requirements in #491. DownloadAbortHandle wraps tokio::task::AbortHandle and aborts the task on drop. DecodedQueuePlayback stores the handle with the fetch_id state…
Out of Scope Changes check	✅ Passed	The reported changes are limited to the shared queue playback state, download-task cancellation, the three linked queue lanes, and the related regression test. Each change supports #491.

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Actionable comments posted: 1

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Inline comments:
In `@src/infra/queue/mod.rs`:
- Line 485: Update publish_decoded to initialize the required abort_handle field
when constructing DecodedQueuePlayback, preserving the expected
Option<DownloadAbortHandle> value for local-files combined with subsonic, qobuz,
or youtube playback.

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• src/infra/queue/mod.rs

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[LargeModGames]

Thanks for the PR, the design is the right one. A Drop on the slot field covers every site that replaces or clears queue_now, the second lock after the spawn closes the race with a fast skip, and finish_decoded_fetch updates the slot in place so the handle survives a completed download. Four things block CI, and two are cleanup.

CI failures

1. Rustfmt. The blank line after each tokio::spawn block carries two spaces, and one new line is over the width limit. cargo fmt --all corrects both.
2. Gates ratchet. The new #[tokio::test] moves test_attribute_total from 1862 to 1863. Bump the value in tools/gates.count in this PR; the ratchet only lets it rise, so this is expected.
3. Clippy (empty_line_after_doc_comments). DownloadAbortHandle landed between the doc comment of DecodedQueuePlayback and its #[cfg], so the doc comment now documents the wrong struct. Move the new struct and its Drop impl above that doc comment and give it a one-line doc of its own. Only the macOS leg and Clippy (all-sources) catch this, because the other legs do not enable subsonic, qobuz, or youtube.
4. The test opens an audio device. LocalPlayer::new() opens the default output, and CI runners have none, so Test Suite (all-sources) and Coverage panic with opening default audio output device. The test also stores the handle by hand, so it only proves that Drop calls abort. A test of DownloadAbortHandle's Drop alone, next to the struct in mod.rs, proves the same thing with no player. Please also drop the test_ prefix; test names in this repo are behavior sentences, for example dropping_the_abort_handle_cancels_the_download_task.

Cleanup

• The three identical blocks after the spawn can be one async fn attach_abort_handle(app, fetch_id, handle). Inside it, the injected flag and the abort_handle.clone() are not needed: store the handle when the fetch_id matches, else abort it.
• The app to app_clone rename is churn. Keep app and give only the copy moved into the closure a new name.

Once these land I will run the full build locally and merge.