Mask timecode in phase refine and guard cutter scene starts

src/cv/global_scan.py

@@ -206,7 +206,12 @@ def _hires_phase_feature(frame: np.ndarray) -> np.ndarray:
     with an 8×8 spatial histogram grid provides enough spatial resolution to
     discriminate facial expression phases within a single continuous scene.
     """
-    trimmed = _trim_dark_borders(frame)
+    trimmed = _trim_dark_borders(frame).copy()
+    h0, w0 = trimmed.shape[:2]
+    # Source and trailer masters often contain burned-in timecode in the
+    # upper-left corner.  It changes every frame and can dominate fine phase
+    # matching, so neutralise that area before extracting hi-res features.
+    trimmed[: int(h0 * 0.16), : int(w0 * 0.28)] = 0
     h, w = trimmed.shape[:2]
     cropped = trimmed[int(h * 0.05):int(h * 0.95), :]
     gray = cv2.cvtColor(cropped, cv2.COLOR_BGR2GRAY)