refactor: cap waveform decode length + guard frombuffer + tighten test

core/waveform.py

@@ -32,4 +32,6 @@ def load_region_samples(path: str, start: float, duration: float,
         return np.zeros(0, dtype="float32")
     if proc.returncode != 0 or not proc.stdout:
         return np.zeros(0, dtype="float32")
-    return np.frombuffer(proc.stdout, dtype="float32")
+    buf = proc.stdout
+    buf = buf[: len(buf) - (len(buf) % 4)]
+    return np.frombuffer(buf, dtype="float32")