- Add VideoOp base class and SliceOp in _input/video_types.py
- Add sliced() method to VideoInput that returns a copy with operation appended
- Each subclass applies operations in get_components() and get_frame_count()
- After materialization, VideoFromFile delegates to internal VideoFromComponents
- Add VideoSlice node that uses video.sliced(start_frame, frame_count)
- Add tests for SliceOp, sliced() behavior, and materialization