feat: SelvaFeatureExtractor outputs prompt as STRING

Users can now wire the prompt output directly to SelvaSampler's prompt input,
making the data flow explicit instead of relying on the implicit features fallback.

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

nodes/selva_feature_extractor.py

@@ -65,8 +65,8 @@ class SelvaFeatureExtractor:
             },
         }
 
-    RETURN_TYPES = ("SELVA_FEATURES", "FLOAT")
-    RETURN_NAMES = ("features", "fps")
+    RETURN_TYPES = ("SELVA_FEATURES", "FLOAT", "STRING")
+    RETURN_NAMES = ("features", "fps", "prompt")
     FUNCTION = "extract_features"
     CATEGORY = PRISMAUDIO_CATEGORY
 
@@ -92,7 +92,8 @@ class SelvaFeatureExtractor:
 
         if os.path.exists(cached_path):
             print(f"[SelVA] Using cached features: {cached_path}", flush=True)
-            return (_load_cached(cached_path), float(fps))
+            cached = _load_cached(cached_path)
+            return (cached, float(fps), cached.get("prompt", prompt))
 
         device   = get_device()
         dtype    = model["dtype"]
@@ -159,7 +160,7 @@ class SelvaFeatureExtractor:
             "sync_features": sync_features.cpu(),
             "duration": float(duration),
             "prompt": prompt,
-        }, float(fps))
+        }, float(fps), prompt)
 
 
 def _load_cached(path):