fix(amazon): add set_engine/set_shutdown_check so _engine gets wired

AmazonDownloadClient was missing set_engine() and set_shutdown_check().
The download engine auto-wires plugins by calling set_engine(self) at
registration time if the method exists (engine.py:136). Without it,
_engine stayed None forever, causing every download() call to raise
RuntimeError("_engine is not set") — silently failing and marking all
tracks not found.

All other streaming clients (Deezer, Qobuz, Tidal, HiFi, SoundCloud)
expose set_engine(); Amazon now matches the pattern.

Tests added: set_engine wires _engine, set_shutdown_check wires callback,
set_engine unblocks download dispatch (the exact live failure mode).

core/amazon_download_client.py

@@ -95,6 +95,13 @@ class AmazonDownloadClient(DownloadSourcePlugin):
     # DownloadSourcePlugin — lifecycle
     # ------------------------------------------------------------------
 
+    def set_engine(self, engine) -> None:
+        """Engine callback — wires the central thread worker + state store."""
+        self._engine = engine
+
+    def set_shutdown_check(self, check_callable) -> None:
+        self.shutdown_check = check_callable
+
     def is_configured(self) -> bool:
         # T2Tunes has a public default instance; no credentials required.
         # Return True unconditionally so the source shows as available.

tests/tools/test_amazon_download_client.py

@@ -679,6 +679,29 @@ class TestDownloadDispatch:
         assert call[1]["target_id"] == "B09XYZ1234"
         assert call[1]["display_name"] == "Artist - Title"
 
+    def test_set_engine_wires_engine(self, tmp_path):
+        client = _make_client(tmp_path)
+        assert client._engine is None
+        engine = MagicMock()
+        client.set_engine(engine)
+        assert client._engine is engine
+
+    def test_set_shutdown_check_wires_callback(self, tmp_path):
+        client = _make_client(tmp_path)
+        assert client.shutdown_check is None
+        check = lambda: False
+        client.set_shutdown_check(check)
+        assert client.shutdown_check is check
+
+    def test_set_engine_allows_download_dispatch(self, tmp_path):
+        """set_engine() must unblock download() — the live failure mode."""
+        client = _make_client(tmp_path)
+        engine = MagicMock()
+        engine.worker.dispatch.return_value = "dl-wired"
+        client.set_engine(engine)
+        result = run(client.download("amazon", "B09XYZ1234||Artist - Title"))
+        assert result == "dl-wired"
+
 
 # ---------------------------------------------------------------------------
 # Status interface