Scoped dotnet format removed the unused entityIndex parameter from
TraversedBackwards_SquareCornersStayConvex, leaving MemberData supplying
two values to a one-parameter theory (8 xUnit failures at test discovery).
The parameter is now genuinely used: each geometric corner is classified
from both adjacent entities of the reversed contour.
Expose TryClassifyAutomaticStartCorner: an internal read-only query over
the emitter's existing TryGetCorner/ClassifyCorner with the same winding
derivation EmitContour uses, so start-point planning can prefer convex
corners without copying tangent math or touching lead generation. Corner-
kind characterization covers convex, reflex, tangent-smooth and cusp
vertices from either adjacent edge, both windings, under rotation, and
rejects midpoints and open contours.