Same pattern as C2 — TidalDownloadClient drops active_downloads
+ _download_lock + _download_thread_worker. download() delegates
to engine.worker.dispatch with _download_sync as the impl.
Source-specific extras (track_id, display_name) merge into the
engine record.
The HLS-segment progress callback (_update_download_progress)
now writes to engine state via engine.update_record instead of
mutating the per-client dict in-place.
Query/cancel methods (get_all_downloads, get_download_status,
cancel_download, clear_all_completed_downloads) now read engine
state via the same accessors as the YouTube migration.
Pinning tests updated to assert engine state. Suite still green
(313 download tests). Behavior preserved end-to-end.
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