Adds a 4th checkbox 'Punti SAM21 (gialli)' that draws the full SAM21 segmentation (input to both the plane partition and Open3DModel) on the RGB result canvas. The backend now returns sam21_points_px and n_sam21 alongside the existing accepted/excluded splits. Useful as a sanity check: visualise what was actually segmented vs what the plane filter kept/rejected vs where the STL got placed. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>