Fillets along a straight edge between two cylinders: trim curved cap edges to the contacts (#238)

* Fillets along a straight edge between two cylinders: the rolling-ball blend trims curved cap edges to its contacts

The edge where two overlapping extruded arcs meet is a straight line between two cylinders
ending on planar caps. The rolling-ball blend built its sections and surface, but only
straight seams could be shortened to the contacts, so a circular cap edge was refused with
"a curved edge where the blend ends". Curved seams now keep the part of their curve between
the new ends, found from the nearest of a few samples (a search from the far end of a long
arc can settle on the opposite side of the circle).

The ball's surface projections start from each face's last answer instead of a cold search,
falling back to one when the warm answer leaves the surface's parameters or is farther than
the last point: about 4.5x faster per fillet (370 -> 80 ms), so dragging the radius is usable.

Fixes #237

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Curve blend: refuse a closed curved seam where the blend ends

A seam that starts and ends at the same vertex (a full circle) has no
"between" two contacts, so trimming it built a broken edge. Return
"not supported yet" instead. The curved-seam trim moves into
trim_curved_seam so it can be tested directly.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: bherbruck <brennen.herbruck@gmail.com>

ROADMAP.md

@@ -160,7 +160,8 @@ headline counts in-scope tabs only and lists the deferred ones separately.
   convex planar body or of an extruded part; fillets inside patterns; closed chains of curved
   edges between any two smooth sides (a branch pipe on a main pipe), by a rolling ball (the
   blend a rational B-spline through exact arcs), closed or ending at planar faces square to the
-  edges.
+  edges, where straight or curved cap edges are trimmed to the contacts (the straight edge
+  between two overlapping extruded arcs).
 - Booleans: prisms along one direction (plates with holes, slots, patterns of them, equal
   sections stacked end to end) are combined exactly in 2D. Qualified axial cuts preserve
   annular holes with overrun cutters, including inward Press Pull after a joined extrusion;