fix(worker): retrieve Soulseek downloads from slskd's dir into staging

SlskdClient enqueued the transfer and polled to Completed but never moved any
files into `dest` — slskd saves them to its OWN downloads dir, so staging stayed
empty and every Soulseek job failed "incomplete download". Qobuz/YouTube write
straight into staging and were unaffected.

- SlskdClient._retrieve() moves the finished files out of slskd's downloads dir
  into staging after completion, matching tolerantly by basename and preferring
  a parent-folder + size match (slskd's on-disk layout varies by version).
- download() returns the moved count as track_count so the pipeline's
  completeness check reflects what actually landed; raises loudly if nothing was
  found (misconfigured/unmounted downloads dir).
- Downloads dir resolves slskd.downloadsDir Config > SLSKD_DOWNLOADS_ROOT env >
  /slskd-downloads default; docker-compose mounts ${SLSKD_DOWNLOADS_DIR} there.
- .env.example documents SLSKD_DOWNLOADS_DIR; retrieval unit-tested offline.

slskd runs as an external daemon, so the shared dir must be a host path both
slskd and the worker mount — set once Lyra runs on the slskd host. Not yet
verified live end-to-end (no slskd access from this VM).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

worker/tests/test_slskd_retrieve.py

@@ -0,0 +1,73 @@
+"""Offline tests for SlskdClient._retrieve — the step that moves slskd's finished
+downloads out of its own dir into the pipeline's staging dir. The search/download REST
+flow stays live-only (test_soulseek_live.py); retrieval is pure filesystem, so it is
+unit-testable here."""
+import os
+
+from lyra_worker.adapters._slskd import SlskdClient
+
+
+def _client(downloads_dir: str) -> SlskdClient:
+    return SlskdClient({"slskd.url": "http://x", "slskd.api_key": "k",
+                        "slskd.downloadsDir": downloads_dir})
+
+
+def _write(path: str, content: bytes = b"audio") -> None:
+    os.makedirs(os.path.dirname(path), exist_ok=True)
+    with open(path, "wb") as fh:
+        fh.write(content)
+
+
+def test_retrieve_moves_files_from_slskd_dir(tmp_path):
+    downloads = tmp_path / "slskd"
+    dest = tmp_path / "staging"
+    # slskd's typical layout: <downloads>/<remote parent folder>/<file>
+    _write(str(downloads / "Artist - Album" / "01 One.flac"))
+    _write(str(downloads / "Artist - Album" / "02 Two.flac"))
+    files = [
+        {"filename": r"@@abc\music\Artist - Album\01 One.flac", "size": 5},
+        {"filename": r"@@abc\music\Artist - Album\02 Two.flac", "size": 5},
+    ]
+
+    moved = _client(str(downloads))._retrieve(files, str(dest))
+
+    assert moved == 2
+    assert sorted(os.listdir(dest)) == ["01 One.flac", "02 Two.flac"]
+    # the source files were moved, not copied
+    assert os.listdir(downloads / "Artist - Album") == []
+
+
+def test_retrieve_disambiguates_by_parent_and_size(tmp_path):
+    downloads = tmp_path / "slskd"
+    dest = tmp_path / "staging"
+    # a decoy with the same basename in a different folder + wrong size
+    _write(str(downloads / "Other Album" / "01 One.flac"), b"wrongsize!!")
+    _write(str(downloads / "Artist - Album" / "01 One.flac"), b"right")
+    files = [{"filename": r"x\Artist - Album\01 One.flac", "size": len(b"right")}]
+
+    moved = _client(str(downloads))._retrieve(files, str(dest))
+
+    assert moved == 1
+    with open(dest / "01 One.flac", "rb") as fh:
+        assert fh.read() == b"right"  # picked the parent+size match, not the decoy
+
+
+def test_retrieve_returns_zero_when_nothing_matches(tmp_path):
+    downloads = tmp_path / "slskd"
+    downloads.mkdir()
+    dest = tmp_path / "staging"
+    files = [{"filename": r"x\Album\missing.flac", "size": 1}]
+
+    assert _client(str(downloads))._retrieve(files, str(dest)) == 0
+
+
+def test_retrieve_raises_when_downloads_dir_absent(tmp_path):
+    dest = tmp_path / "staging"
+    client = _client(str(tmp_path / "does-not-exist"))
+    files = [{"filename": r"x\Album\a.flac", "size": 1}]
+    try:
+        client._retrieve(files, str(dest))
+    except RuntimeError as e:
+        assert "downloads dir not found" in str(e)
+    else:
+        raise AssertionError("expected RuntimeError for a missing downloads dir")