Same pattern as C2/C3/C4. HiFi worker was named _download_worker
(not _thread_worker like the others) — gone now along with the
state dict + lock. Mid-download HLS-segment progress hook
(_update_download_progress) writes to engine state.
Pinning tests updated. Suite still green (318 download tests).
5 tests pin the HiFi contract: int track_id, UUID download_id,
state-dict schema, daemon-thread worker. Note: target method is
`_download_worker` (NOT `_thread_worker` like Tidal/Qobuz) and
worker signature is 3-arg (download_id, track_id, display_name).
Engine refactor's plugin contract must accommodate or normalize.