A6: Pin DeezerDownloadClient download lifecycle behavior

6 tests pin the Deezer contract:
- track_id stays as STRING (Deezer GW API uses string IDs).
- username slot is the legacy `'deezer_dl'` (frontend depends on it).
- Auth gate at top of `download()` returns None BEFORE thread spawn.
- Defensive fallback: filename without `||` synthesizes display name.
- Thread is named `deezer-dl-<track_id>` for diagnostics.
- State dict has Deezer-specific `error` slot.

tests/downloads/test_deezer_pinning.py

@@ -0,0 +1,131 @@
+"""Phase A pinning tests for DeezerDownloadClient's download lifecycle.
+
+Deezer auths via ARL token, fetches Blowfish-encrypted FLAC chunks
+from the Deezer GW API, decrypts client-side. Different from
+Tidal/Qobuz/HiFi:
+
+- track_id is STRING (not int).
+- username is the legacy ``'deezer_dl'`` (not ``'deezer'``).
+- Auth gate at the top of `download()` short-circuits when not
+  authenticated (returns None without spawning a thread).
+- Thread is named ``deezer-dl-<track_id>`` for diagnostics.
+
+Engine refactor must preserve all of these.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import threading
+from pathlib import Path
+from unittest.mock import patch
+
+import pytest
+
+from core.deezer_download_client import DeezerDownloadClient
+
+
+def _run_async(coro):
+    loop = asyncio.new_event_loop()
+    try:
+        return loop.run_until_complete(coro)
+    finally:
+        loop.close()
+
+
+@pytest.fixture
+def deezer_client():
+    client = DeezerDownloadClient.__new__(DeezerDownloadClient)
+    client.download_path = Path('./test_deezer_downloads')
+    client.shutdown_check = None
+    client.active_downloads = {}
+    client._download_lock = threading.Lock()
+    client._authenticated = True
+    return client
+
+
+def test_download_returns_none_when_not_authenticated(deezer_client):
+    """Pinning: unauthenticated client refuses BEFORE any thread is
+    spawned. The orchestrator's hybrid fallback depends on this
+    early return — if the auth gate moves into the thread, fallback
+    behavior changes."""
+    deezer_client._authenticated = False
+    result = _run_async(deezer_client.download('deezer_dl', '12345||Some Song', 0))
+    assert result is None
+
+
+def test_download_accepts_string_track_id(deezer_client):
+    """Pinning: Deezer track_id stays as string — the GW API uses
+    string IDs. Engine refactor cannot int-coerce on the way through."""
+    with patch('core.deezer_download_client.threading.Thread') as fake:
+        fake.return_value.start = lambda: None
+        download_id = _run_async(
+            deezer_client.download('deezer_dl', '999||My Deezer Song', 5000)
+        )
+
+    record = deezer_client.active_downloads[download_id]
+    assert record['track_id'] == '999'  # STRING, not int
+    assert isinstance(record['track_id'], str)
+
+
+def test_download_username_field_is_legacy_deezer_dl(deezer_client):
+    """Pinning: the `username` slot in the state dict is ``'deezer_dl'``,
+    not ``'deezer'``. Frontend status indicators + per-source
+    dispatch strings depend on the legacy form."""
+    with patch('core.deezer_download_client.threading.Thread') as fake:
+        fake.return_value.start = lambda: None
+        download_id = _run_async(
+            deezer_client.download('deezer_dl', '999||x', 0)
+        )
+
+    assert deezer_client.active_downloads[download_id]['username'] == 'deezer_dl'
+
+
+def test_download_handles_missing_display_name_with_fallback(deezer_client):
+    """Pinning: filename without `||` produces a synthetic display
+    name `Track <track_id>`. Other clients return None for missing
+    `||` — Deezer is more lenient. Engine refactor must NOT change
+    this defensive fallback."""
+    with patch('core.deezer_download_client.threading.Thread') as fake:
+        fake.return_value.start = lambda: None
+        download_id = _run_async(deezer_client.download('deezer_dl', '12345', 0))
+
+    assert download_id is not None
+    record = deezer_client.active_downloads[download_id]
+    assert record['display_name'] == 'Track 12345'
+
+
+def test_download_populates_active_downloads_with_initial_state(deezer_client):
+    """Pinning: per-download record schema. NOTE the extra `error`
+    slot — Deezer-specific, used for ARL re-auth failure messages."""
+    with patch('core.deezer_download_client.threading.Thread') as fake:
+        fake.return_value.start = lambda: None
+        download_id = _run_async(
+            deezer_client.download('deezer_dl', '999||My Deezer Song', 1024)
+        )
+
+    record = deezer_client.active_downloads[download_id]
+    assert record['id'] == download_id
+    assert record['filename'] == '999||My Deezer Song'
+    assert record['username'] == 'deezer_dl'
+    assert record['state'] == 'Initializing'
+    assert record['size'] == 1024  # Deezer respects the file_size hint
+    assert record['file_path'] is None
+    assert record['error'] is None  # Deezer-specific slot
+
+
+def test_download_thread_is_named_for_diagnostics(deezer_client):
+    """Pinning: thread is named `deezer-dl-<track_id>` so multi-thread
+    debugging shows which download a stuck thread belongs to. Engine
+    refactor's BackgroundDownloadWorker must preserve diagnostic naming."""
+    captured_kwargs = {}
+
+    def capture_thread(*args, **kwargs):
+        captured_kwargs.update(kwargs)
+        return type('FakeThread', (), {'start': lambda self: None})()
+
+    with patch('core.deezer_download_client.threading.Thread', side_effect=capture_thread):
+        _run_async(deezer_client.download('deezer_dl', '777||Title', 0))
+
+    assert captured_kwargs.get('daemon') is True
+    assert captured_kwargs.get('name') == 'deezer-dl-777'