Refactor tool-speechtotext: extract sttlib shared library and add tests

Extract duplicated code (Whisper loading, audio recording, transcription,
VAD processing) into reusable sttlib/ package. Rewrite all 3 scripts as
thin wrappers. Add 24 unit tests with mocked hardware. Fix GPU fallback
bug in assistant.py and args.system assignment bug.

python/tool-speechtotext/sttlib/whisper_loader.py

@@ -0,0 +1,15 @@
+import os
+from faster_whisper import WhisperModel
+
+os.environ["CT2_CUDA_ALLOW_FP16"] = "1"
+
+
+def load_whisper_model(model_size):
+    """Load Whisper with GPU (cuda/float16) -> CPU (cpu/int8) fallback."""
+    print(f"Loading Whisper model ({model_size})...")
+    try:
+        return WhisperModel(model_size, device="cuda", compute_type="float16")
+    except Exception as e:
+        print(f"GPU loading failed: {e}")
+        print("Falling back to CPU (int8)")
+        return WhisperModel(model_size, device="cpu", compute_type="int8")