CV Find Quadrilateral

Finds the largest convex quadrilateral among the contours - the classic document/whiteboard outline detector. Each contour (largest first) is simplified with cv2.approxPolyDP; the first one that simplifies to 4 convex corners and covers at least min_area_fraction of the image wins. When that strict pass finds nothing, a second pass simplifies each contour's CONVEX HULL instead, raising the tolerance until 4 corners remain - recovering pages with curled corners, shadow notches or texture-broken edges. The corners come out ordered top-left, top-right, bottom-right, bottom-left, ready for 'CV Quad To Rectangle' + cv2.getPerspectiveTransform. Failure-tolerant: when no quad qualifies it returns found=false and the full image corners (warping with them is a no-op), so the workflow keeps running - branch on 'found' with an if/else node.