Module features 

Event feature detection (OpenCV features2d analogue). Both detectors maintain a per-polarity Surface of Active Events (SAE — the latest timestamp seen at each pixel) and return a new EventStream holding only the events that sit on a moving corner, so detection chains like a denoising filter. Both assume events arrive in ascending time order (what the readers produce); call EventStream::sort_by_time first if a stream might be unordered.