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.
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