Fix filtfilt padlen for 0.3Hz gravity filter, default 9-sample pad inadequate, use max safe pad

src/phyphox_pipeline.py

@@ -83,10 +83,13 @@ def _butter_lp(data: np.ndarray, cutoff: float, fs: float = FS, order: int = 3)
         Filtered array, same shape as input
     """
     b, a = sp_signal.butter(order, cutoff / (fs / 2.0), btype="low")
+    # Use maximum safe padding — critical for low cutoffs (e.g. 0.3 Hz needs
+    # ~167 samples to settle; scipy's default 9-sample pad is far too short).
+    padlen = min(len(data) - 1, max(3 * int(fs / cutoff), 9))
     if data.ndim == 1:
-        return sp_signal.filtfilt(b, a, data)
+        return sp_signal.filtfilt(b, a, data, padlen=padlen)
     return np.column_stack(
-        [sp_signal.filtfilt(b, a, data[:, i]) for i in range(data.shape[1])]
+        [sp_signal.filtfilt(b, a, data[:, i], padlen=padlen) for i in range(data.shape[1])]
     )