fix(cache): guard semantic detector initialization

Author: JerrettDavis

headroom/cache/dynamic_detector.py

@@ -770,14 +770,16 @@ def detect(
         # Encode all sentences
         if not sentences:
             return [], None
+        if self._model is None or self._exemplar_embeddings is None:
+            return [], self._load_error or "semantic detector is not initialized"
 
         try:
             import numpy as np
         except ImportError:
             return [], "numpy not installed. Install with: pip install numpy"
 
         sentence_texts = [s[0] for s in sentences]
-        sentence_embeddings = self._model.encode(  # type: ignore[union-attr]
+        sentence_embeddings = self._model.encode(
             sentence_texts,
             convert_to_numpy=True,
         )