Pin opencv-python <4.13 for Intel macOS due to VideoWriter regression

opencv-python 4.13.0.90 was released on PyPI on January 18, 2026 and
introduced a regression in cv2.VideoWriter functionality on Intel macOS.

The VideoWriter.write() method fails silently - files are not created,
causing all video_organize tests to fail with FileNotFoundError:
- test_video_organize[*] (8 variations)
- test_video_organize_common[copy-0]

Evidence:
- Jan 18, 2026 (passing): opencv-python 4.12.0.88
- Jan 19, 2026+ (failing): opencv-python 4.13.0.90
- Same commit (ccdb659), same runner image (macos-15/20260112.0116)
- Only affects macos-15-intel platform
- All 9 test failures are in video organization tests
- Error: test video files created by cv2.VideoWriter fixture don't exist

The constraint is platform-specific (Intel macOS only) to allow:
- ARM macOS: use latest opencv-python
- Linux/Windows: use latest opencv-python
- Intel macOS: pin to 4.12.x until upstream fix

Upstream tracking: https://github.com/opencv/opencv-python/releases

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: yarikoptic

pyproject.toml

@@ -101,7 +101,9 @@ style = [
 test = [
     "anys ~= 0.2",
     "coverage",
-    "opencv-python",
+    # Workaround for VideoWriter regression in 4.13.0.90 on Intel macOS
+    "opencv-python < 4.13 ; sys_platform == 'darwin' and platform_machine != 'arm64'",
+    "opencv-python ; sys_platform != 'darwin' or platform_machine == 'arm64'",
     "pytest",
     "pytest-cov",
     "pytest-mock",