Consider a newtype for contours

Currently we're using `BezPath` directly, but a contour has more explicit constraints than a BezPath; most importantly BezPath supports the idea of multiple subpaths, whereas a contour is always a single subpath.

This does not feel high priority, but worth capturing as a future ergonomics improvement.

No files, tests, or entry points are named. Start by tracing where BezPath is used for contours and identify the API constraints a contour must enforce. Done means reaching a decided design for a contour-specific type that cannot represent multiple subpaths, with affected uses and tests identified.

Written by the indexing model from the issue text.