fix: return fps from non-cache extraction path

The fps output was only returned on cache hits. Fresh extractions
returned only features, leaving fps null.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nodes/feature_extractor.py

@@ -203,4 +203,5 @@ class PrismAudioFeatureExtractor:
 
         # Load the extracted features
         loader = PrismAudioFeatureLoader()
-        return loader.load_features(cached_path)
+        features, = loader.load_features(cached_path)
+        return (features, float(fps))