SU.747c NURBS surfaces, step S3: fillets and blends between faces, thickening, a solid written to STEP as one closed shell

Asked (#747, #748, #749, #750, Oct 16): concept-to-prototype NURBS and production-quality surfacing. S1 (curves, untrimmed surfaces, measured G0 to G2) is Built in 0.8; trimmed surfaces, solids from them with union, subtract and intersect, and PartCraft Bake are Built in /lab 0.9 (docs/0.9-sessions/nurbs-09.md). That log lists what is left and says 'Fillets between faces come next'.

Plan (lib/nurbs): (1) fillets and blends between two faces, constant then variable radius, deviation from the rolling ball measured; (2) thickening a surface into a solid, self-intersection refused and named; (3) a solid written to STEP as one closed shell, read back; (4) Booleans as nodes. Each result measured against a closed form.

Not claimed: class-A surfacing, the other programs' file formats, a result equal to Alias or ICEM Surf. Class-A and equality with Alias/ICEM are never claimed.

Completed 2026-11-06: in /lab (ModelCraft, PartCraft, SuperCad Studio's panel through the shared libs): fillet between two faces (constant/variable radius, convex/concave, closed edge; deviation from the rolling ball measured), several edges at once (mitred corners; one sphere patch where three flat square faces meet, radius equal), thicken into a solid, STEP and IGES (186 over closed shell 514) with a solid as one closed shell, read back as a solid, Booleans as nodes in the node editor, help lines. Measured against closed forms (removed volume within 2 % at tol 0.05, goes with the tolerance). Not built: vertex blend for curved faces or unequal radii, fillet between faces that do not meet, inside fillet ending on a slanted or curved face, several inside edges that reliably close (the result line says if not), Booleans/fillets on the surfaces without triangles, IGES not tried in another program. Tests: docs/0.9-sessions/nurbs-s3-09.md.