refactor(scan): extract scan_chunk from scan_library

scan_chunk resolves album folders whose (artist, album) sorts after a
cursor, up to a limit, returning (new_cursor, imported, skipped, done).
scan_library becomes the unbounded case (limit=None), so its behavior and
tests are unchanged. Idempotent: all writes are ON CONFLICT, so an
overlapping/repeated cursor is safe. Enables chunked scanning in the loop.

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

worker/tests/test_scan_chunk.py

@@ -0,0 +1,81 @@
+from lyra_worker.adapters.fakes import FakeMbBrowser
+from lyra_worker.browser import ReleaseGroupInfo
+from lyra_worker.scan import scan_chunk
+from lyra_worker.types import MBTarget
+from tests.test_scan import FakeProbe, FakeResolver, _album, _counts
+
+
+def _tree(tmp_path):
+    """Three albums across two artists, in nested-sorted order:
+    Artist A/Alpha, Artist A/Beta, Artist B/Gamma."""
+    _album(tmp_path, "Artist A", "Alpha (2001)")
+    _album(tmp_path, "Artist A", "Beta (2002)")
+    _album(tmp_path, "Artist B", "Gamma (2003)")
+    resolver = FakeResolver({
+        ("Artist A", "Alpha"): MBTarget(artist="Artist A", album="Alpha", year=2001, rg_mbid="rg-a1", artist_mbid="ma"),
+        ("Artist A", "Beta"): MBTarget(artist="Artist A", album="Beta", year=2002, rg_mbid="rg-a2", artist_mbid="ma"),
+        ("Artist B", "Gamma"): MBTarget(artist="Artist B", album="Gamma", year=2003, rg_mbid="rg-b1", artist_mbid="mb"),
+    })
+    browser = FakeMbBrowser(releases={
+        "ma": [ReleaseGroupInfo("rg-a1", "Alpha", "Album", (), "2001"),
+               ReleaseGroupInfo("rg-a2", "Beta", "Album", (), "2002")],
+        "mb": [ReleaseGroupInfo("rg-b1", "Gamma", "Album", (), "2003")],
+    })
+    return resolver, browser
+
+
+def test_scan_chunk_respects_limit_and_returns_cursor(conn, tmp_path):
+    resolver, browser = _tree(tmp_path)
+    cursor, imported, skipped, done = scan_chunk(
+        conn, resolver, FakeProbe(quality_class=2), browser, str(tmp_path), cursor="", limit=2)
+    assert (imported, skipped, done) == (2, 0, False)  # processed 2 of 3, more remain
+    assert cursor == "Artist A/Beta (2002)"            # last processed key
+    with conn.cursor() as cur:
+        cur.execute('SELECT count(*) FROM "LibraryItem"')
+        assert cur.fetchone()[0] == 2
+
+
+def test_scan_chunk_resumes_from_cursor(conn, tmp_path):
+    resolver, browser = _tree(tmp_path)
+    cursor, imported, skipped, done = scan_chunk(
+        conn, resolver, FakeProbe(quality_class=2), browser, str(tmp_path),
+        cursor="Artist A/Beta (2002)", limit=25)
+    assert (imported, skipped, done) == (1, 0, True)   # only Gamma remains
+    assert cursor == "Artist B/Gamma (2003)"
+    with conn.cursor() as cur:
+        cur.execute('SELECT album FROM "LibraryItem"')
+        assert cur.fetchall() == [("Gamma",)]          # only Gamma imported
+
+
+def test_scan_chunk_cursor_past_end_is_noop(conn, tmp_path):
+    resolver, browser = _tree(tmp_path)
+    cursor, imported, skipped, done = scan_chunk(
+        conn, resolver, FakeProbe(), browser, str(tmp_path), cursor="zzzz/zzzz", limit=10)
+    assert (imported, skipped, done) == (0, 0, True)
+    assert cursor == "zzzz/zzzz"                        # unchanged
+    assert _counts(conn)["LibraryItem"] == 0
+
+
+def test_scan_chunk_splits_artist_across_chunks_idempotently(conn, tmp_path):
+    # limit=1 forces Artist A's two albums into separate chunks, re-browsing its
+    # discography — which must stay idempotent (ON CONFLICT) and total the same as one scan.
+    resolver, browser = _tree(tmp_path)
+    cursor, done, total_i, total_s, guard = "", False, 0, 0, 0
+    while not done and guard < 10:
+        cursor, i, s, done = scan_chunk(
+            conn, resolver, FakeProbe(quality_class=2), browser, str(tmp_path), cursor=cursor, limit=1)
+        total_i += i
+        total_s += s
+        guard += 1
+    assert (total_i, total_s) == (3, 0)
+    assert guard == 3  # three albums, one per chunk, then done
+    assert _counts(conn) == {"LibraryItem": 3, "MonitoredRelease": 3, "WatchedArtist": 2}
+
+
+def test_scan_chunk_unbounded_matches_full_walk(conn, tmp_path):
+    resolver, browser = _tree(tmp_path)
+    cursor, imported, skipped, done = scan_chunk(
+        conn, resolver, FakeProbe(quality_class=2), browser, str(tmp_path), cursor="", limit=None)
+    assert (imported, skipped, done) == (3, 0, True)
+    assert cursor == "Artist B/Gamma (2003)"
+    assert _counts(conn) == {"LibraryItem": 3, "MonitoredRelease": 3, "WatchedArtist": 2}