player: log last DLNA URL to /tmp/dlna_last_url.txt on play(); tests: auto-load it in diagnostic script

tests/test_video_playback_device.py

@@ -218,11 +218,21 @@ if not audio_ok:
 
 test_url = sys.argv[1] if len(sys.argv) > 1 else None
 
+if not test_url:
+    try:
+        with open("/tmp/dlna_last_url.txt") as _f:
+            test_url = _f.read().strip() or None
+        if test_url:
+            print(f"\n  [auto] Loaded last DLNA URL from /tmp/dlna_last_url.txt")
+    except FileNotFoundError:
+        pass
+
 _section("6. Live URL / file playback")
 
 if not test_url:
     _warn("No URL provided — skipping live playback test.")
-    print("       Pass a media URL or path as the first argument to test real decode.")
+    print("       Pass a media URL or path as the first argument, or start the app and")
+    print("       play something; the URL will be saved to /tmp/dlna_last_url.txt.")
 else:
     print(f"  URL: {test_url}")
     live_frames = 0