Die-cut previews: clip to cut line, no baked effects

Showing a raw print PNG with 3 mm bleed on a review board makes the bleed look like an edit or glitch, so clip each preview to the SVG's CutContour path. Multiply the cut mask with the file's own alpha (ImageChops.multiply), or holes such as a ring interior get filled in. Traced cut paths can have multiple subpaths (outer outline plus inner hole), so use even-odd fill. Don't add drop shadows or halos when the user is judging the originals: reviewers read them as changes to the artwork, even if an existing preview style has them. Prove source files are unchanged with cmp against the originals.