A7: Pin SoundcloudClient download lifecycle behavior

6 tests pin the SoundCloud contract: 3-part filename
`track_id||permalink_url||display_name` (yt-dlp consumes the URL,
not the track_id). Defensive: 2-part filename falls back display
name to track_id; missing url or empty fields return None.
Thread target signature uses URL as the second arg.

tests/downloads/test_soundcloud_pinning.py

@@ -0,0 +1,124 @@
+"""Phase A pinning tests for SoundcloudClient's download lifecycle.
+
+SoundCloud is anonymous-only (no auth required). Uses yt-dlp's
+``scsearch:`` for search, downloads via yt-dlp subprocess. Different
+filename format from every other source: 3-part
+``track_id||permalink_url||display_name`` because yt-dlp needs the
+permalink URL, not the track_id, to actually download.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import threading
+from pathlib import Path
+from unittest.mock import patch
+
+import pytest
+
+from core.soundcloud_client import SoundcloudClient
+
+
+def _run_async(coro):
+    loop = asyncio.new_event_loop()
+    try:
+        return loop.run_until_complete(coro)
+    finally:
+        loop.close()
+
+
+@pytest.fixture
+def sc_client():
+    client = SoundcloudClient.__new__(SoundcloudClient)
+    client.download_path = Path('./test_sc_downloads')
+    client.shutdown_check = None
+    client.active_downloads = {}
+    client._download_lock = threading.Lock()
+    return client
+
+
+def test_download_returns_none_for_filename_with_too_few_parts(sc_client):
+    """Pinning: SoundCloud needs at LEAST `track_id||permalink_url`.
+    A 1-part filename → None. Engine refactor's filename parsing
+    must keep the 2-part minimum."""
+    result = _run_async(sc_client.download('soundcloud', 'just-id-no-url', 0))
+    assert result is None
+
+
+def test_download_returns_none_for_empty_track_id_or_url(sc_client):
+    """Pinning: defensive — if EITHER side of `||` is empty, refuse."""
+    result = _run_async(sc_client.download('soundcloud', '||https://soundcloud.com/x', 0))
+    assert result is None
+    result = _run_async(sc_client.download('soundcloud', 'track123||', 0))
+    assert result is None
+
+
+def test_download_accepts_three_part_filename_with_display(sc_client):
+    """Pinning: 3-part filename `track_id||permalink_url||display`
+    is the canonical form. Display name is extracted as the third
+    field."""
+    with patch('core.soundcloud_client.threading.Thread') as fake:
+        fake.return_value.start = lambda: None
+        download_id = _run_async(sc_client.download(
+            'soundcloud',
+            'sc-12345||https://soundcloud.com/artist/song||Some Display Title',
+            0,
+        ))
+
+    record = sc_client.active_downloads[download_id]
+    assert record['track_id'] == 'sc-12345'
+    assert record['permalink_url'] == 'https://soundcloud.com/artist/song'
+    assert record['display_name'] == 'Some Display Title'
+
+
+def test_download_falls_back_display_name_to_track_id_when_two_part(sc_client):
+    """Pinning: when display name is missing (2-part filename), the
+    track_id IS the display name. Used for some search result
+    encodings that don't carry a separate display."""
+    with patch('core.soundcloud_client.threading.Thread') as fake:
+        fake.return_value.start = lambda: None
+        download_id = _run_async(sc_client.download(
+            'soundcloud', 'sc-99||https://soundcloud.com/x/y', 0,
+        ))
+
+    record = sc_client.active_downloads[download_id]
+    assert record['display_name'] == 'sc-99'
+
+
+def test_download_populates_active_downloads_with_initial_state(sc_client):
+    with patch('core.soundcloud_client.threading.Thread') as fake:
+        fake.return_value.start = lambda: None
+        download_id = _run_async(sc_client.download(
+            'soundcloud', 'sc-1||https://soundcloud.com/x||Title', 0,
+        ))
+
+    record = sc_client.active_downloads[download_id]
+    assert record['id'] == download_id
+    assert record['username'] == 'soundcloud'
+    assert record['state'] == 'Initializing'
+    assert record['progress'] == 0.0
+    assert record['file_path'] is None
+    # Permalink URL stays as a slot — yt-dlp downloads from URL not track_id
+    assert 'permalink_url' in record
+
+
+def test_download_spawns_daemon_thread_with_permalink_url_arg(sc_client):
+    """Pinning: thread target signature is
+    `(download_id, permalink_url, display_name, original_filename)`.
+    Critical: the URL (not the track_id) is what yt-dlp consumes."""
+    captured_kwargs = {}
+
+    def capture_thread(*args, **kwargs):
+        captured_kwargs.update(kwargs)
+        return type('FakeThread', (), {'start': lambda self: None})()
+
+    with patch('core.soundcloud_client.threading.Thread', side_effect=capture_thread):
+        _run_async(sc_client.download(
+            'soundcloud', 'sc-1||https://soundcloud.com/x||Title', 0,
+        ))
+
+    assert captured_kwargs.get('daemon') is True
+    args = captured_kwargs.get('args', ())
+    assert len(args) == 4
+    assert args[1] == 'https://soundcloud.com/x'  # permalink_url, NOT track_id
+    assert args[2] == 'Title'