`BackgroundDownloadWorker` lives on the engine and owns the
boilerplate every streaming download client currently
hand-rolls: thread spawn, per-source semaphore, rate-limit
delay, state lifecycle (Initializing → InProgress → Completed
or Errored), exception capture.
Plugins provide only the atomic download op (`impl_callable`).
Per-source rate-limit policy (concurrency, delay) is configured
on the worker via `set_concurrency` / `set_delay`. Source-
specific record fields merge in via `extra_record_fields` so
existing consumer code that reads `video_id`, `track_id`,
`permalink_url`, etc. keeps working post-migration. Username
slot supports override (Deezer's legacy `'deezer_dl'`).
Phase C1 scope: worker exists. No client migrated yet — C2-C7
migrate sources one at a time, each gated by the Phase A
pinning tests so per-source contract drift fails fast.
10 new tests pin the worker contract: UUID id format, initial
record shape, extra-fields merge, username override, state
transitions on success / impl-returns-None / impl-raises,
semaphore serialization (default + parallel), rate-limit
delay between successive downloads.
Suite still green (308 download tests). Pure additive.
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