Worker did two separable jobs. Both become mixins, because the tests drive
their methods as bound attributes (worker._download_one(), worker._fetch_thread)
and free functions would have broken that:
_scan.py ScanMixin -- threads.json, the archive, one changed thread
_fetch.py DownloadMixin -- claim a file, fetch it, record the outcome
_worker_base.py holds WorkerBase(ABC) with the state both halves touch and the
three methods Worker supplies. Two earlier shapes were abandoned rather than
suppressed: NotImplementedError stubs are uncovered lines under the 100% gate,
and `if TYPE_CHECKING` stubs satisfy mypy but leave pylint reporting no-member.
The ABC satisfies both with no disable comment anywhere.
_downloaded needs a real class-level default: _fetch does `self._downloaded += 1`
and pylint only accepts an attribute as assignable when there is something to
assign to. Worker.__init__ still owns the instance value.
downloader.py 420 -> 238, _scan.py 134, _fetch.py 98, _worker_base.py 52.
277 tests pass.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011w2Pj4Ek3ZNMQVYcFHgXDY
