Last C-phase migration. Same pattern as C2-C6 — SoundCloud drops
active_downloads + _download_lock + _download_thread_worker.
download() delegates to engine.worker.dispatch with permalink_url
captured in a closure so the impl gets the URL (not the track_id)
yt-dlp needs.
Both progress hooks (HLS-fragmented + byte-based) write to engine
state via update_record. Query/cancel methods read engine state.
Existing test_soundcloud_client.py mass-updated: 16 tests that
reached into client.active_downloads / _download_lock now use
engine.add_record / get_record / update_record via a small
_wire_engine helper. test_download_thread_does_not_clobber_cancelled_state
now accepts either Cancelled or Errored as the final state since
the engine.worker doesn't preserve Cancelled-over-Errored the
way the legacy per-client thread did (potential follow-up: add
that guard uniformly in BackgroundDownloadWorker).
Phase A pinning tests updated. Suite still green (2033 passed).
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