Curvature-continuous (G2) fillets for straight planar edges (#862)

Fillet gets a Curvature (G2) radius type (type: "curvature", alias "g2"): a quintic blend whose size is the equivalent circular setback, for convex and concave straight edges between planar faces with a perpendicular planar end face at each end. Unsupported cases are refused rather than falling back to a circular fillet, and a blend that would cross a loop in its end face is refused. Persisted as a fillet style under format 19; labels in en/de/ja/ko/zh.

Co-authored-by: midasdf <midasdf@users.noreply.github.com>

ROADMAP.md

@@ -207,6 +207,9 @@ PCB / Package, Render, Animation, Simulation, Manufacture/CAM, Drawings.
 
 - Associative shell-face sketches require a unique certified planar source. Nested shell
   attachments and ambiguous coincident sources remain unsupported.
+- G2 fillets: exact quintic blends of straight edges between planar faces, with one perpendicular
+  planar end face at each end. The UI and `solid.fillet` expose `curvature` (alias `g2`); the size
+  is an equivalent circular setback. Curved faces/edges, sloped ends and corner junctions remain unsupported.
 - Fillets and chamfers: constant, variable-radius and chord-length fillets; a radius per edge,
   rounds next to earlier rounds of another radius, and smooth corner patches where three rounds
   meet (a sphere for equal radii); straight edges (convex or concave) between planar faces with perpendicular