Split Face records the new edges as polylines on the body (Body::split_lines); the document carries them through later features. STEP export no longer joins pieces of one surface across such a line, and healing (fillets, chamfers, booleans, shell, draft) no longer merges coplanar pieces there, while incidental pieces (boolean leftovers, seam halves) still merge as before. Tests: plane and cylinder splits keep their face count through a STEP round trip (and join without the record), a second split keeps both lines, and an engine test exports a split box after a fillet with 8 faces.
Fixes#11
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT
blend_corner.rs rebuilds the planar faces at the ends of the filleted edges (three planar faces and three straight edges at each such vertex) and keeps the rest. One round at a vertex ends in the third face's exact section of its cylinder; two rounds with the third edge sharp trim each other (rolling ball: the crease from where their contact lines cross to the third edge for equal radii, else to the smaller round's contact line on the face that ends the larger one, which gets the exact section between); three rounds meet a sphere for equal radii, else a rational degree-4 patch through plane sections of the cylinders, the sections turned so that the cylinders' curvatures agree where two meet on a face and the patch fitted tangent to all three (within 0.01 degrees) and to the face at its closed corner.
kernel::fillet falls back to it when the local blends refuse: an edge that ends on an earlier round (any radius) takes the round off (delete_faces) and blends both edges together, so r3 next to r2 and a third edge into a mitred pair work. delete_faces: a gap stretch along several deleted faces keeps the corners between them (two rounds meeting in a mitre come off back to the sharp corner). splitface::fit is shared for the crease.
Tests: volumes against the strips-minus-overlap reference (exact strips, overlap integrated; checked against an independent 1D integral), the sphere-octant formula for three equal rounds, tangency along the patch, closed meshes, refusals without panics.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
CypherNaugh_0x