A5: Pin HiFiClient download lifecycle behavior

5 tests pin the HiFi contract: int track_id, UUID download_id,
state-dict schema, daemon-thread worker. Note: target method is
`_download_worker` (NOT `_thread_worker` like Tidal/Qobuz) and
worker signature is 3-arg (download_id, track_id, display_name).
Engine refactor's plugin contract must accommodate or normalize.

tests/downloads/test_hifi_pinning.py

@@ -0,0 +1,91 @@
+"""Phase A pinning tests for HiFiClient's download lifecycle.
+
+HiFi uses public hifi-api instances backed by Tidal-sourced metadata.
+Same int track_id + thread-worker + state-dict pattern as Tidal/Qobuz,
+EXCEPT the worker method is named `_download_worker` (no `_thread_`).
+Engine refactor must preserve the worker target signature.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import threading
+from pathlib import Path
+from unittest.mock import patch
+
+import pytest
+
+from core.hifi_client import HiFiClient
+
+
+def _run_async(coro):
+    loop = asyncio.new_event_loop()
+    try:
+        return loop.run_until_complete(coro)
+    finally:
+        loop.close()
+
+
+@pytest.fixture
+def hifi_client():
+    client = HiFiClient.__new__(HiFiClient)
+    client.download_path = Path('./test_hifi_downloads')
+    client.shutdown_check = None
+    client.active_downloads = {}
+    client._download_lock = threading.Lock()
+    return client
+
+
+def test_download_returns_none_for_invalid_filename_format(hifi_client):
+    result = _run_async(hifi_client.download('hifi', 'no-separator', 0))
+    assert result is None
+
+
+def test_download_returns_none_for_non_integer_track_id(hifi_client):
+    result = _run_async(hifi_client.download('hifi', 'not-int||title', 0))
+    assert result is None
+
+
+def test_download_returns_uuid_for_valid_filename(hifi_client):
+    with patch('core.hifi_client.threading.Thread') as fake:
+        fake.return_value.start = lambda: None
+        result = _run_async(hifi_client.download('hifi', '12345||Some Song', 0))
+    assert result is not None
+    assert len(result) == 36
+
+
+def test_download_populates_active_downloads_with_initial_state(hifi_client):
+    with patch('core.hifi_client.threading.Thread') as fake:
+        fake.return_value.start = lambda: None
+        download_id = _run_async(hifi_client.download('hifi', '999||My HiFi Song', 0))
+
+    record = hifi_client.active_downloads[download_id]
+    assert record['id'] == download_id
+    assert record['filename'] == '999||My HiFi Song'
+    assert record['username'] == 'hifi'
+    assert record['state'] == 'Initializing'
+    assert record['track_id'] == 999
+    assert record['display_name'] == 'My HiFi Song'
+
+
+def test_download_spawns_daemon_thread_targeting_download_worker(hifi_client):
+    """Pinning: target is `_download_worker` (NOT `_thread_worker` like
+    Tidal/Qobuz). Engine refactor's plugin contract must accommodate
+    this naming variance OR force a rename — pinned here so the
+    decision is conscious."""
+    captured_kwargs = {}
+
+    def capture_thread(*args, **kwargs):
+        captured_kwargs.update(kwargs)
+        return type('FakeThread', (), {'start': lambda self: None})()
+
+    with patch('core.hifi_client.threading.Thread', side_effect=capture_thread):
+        _run_async(hifi_client.download('hifi', '777||Title', 0))
+
+    assert captured_kwargs.get('daemon') is True
+    assert captured_kwargs.get('target') == hifi_client._download_worker
+    args = captured_kwargs.get('args', ())
+    # HiFi's worker signature: (download_id, track_id, display_name) — 3 args, not 4
+    assert len(args) == 3
+    assert args[1] == 777
+    assert args[2] == 'Title'