Extend any surface: natural, tangent and curvature, up to a target (#760) (#1166)

Extend works on any surface: natural (continuing the face's own surface), tangent (G1 run-off) or curvature (G2 run-off), by a distance, up to a plane or face, or back to the surface's bounds, with neighbouring edges lengthened to meet. Untrim and Extend share face parameter-loop helpers. The new SurfaceExtend fields are noted under format 21. Closes #760.

crates/doc/src/feature_walk_tests/fixtures.rs

@@ -153,6 +153,14 @@ pub(super) fn fixtures() -> Vec<(&'static str, Value)> {
         ("reverse_normal", json!({"type": "reverse_normal", "body": "B"})),
         ("surface_untrim", json!({"type": "surface_untrim", "face": {"body": "B", "point": o}, "holes": [o], "outer": true})),
         ("surface_extend", json!({"type": "surface_extend", "body": "B", "edges": [o], "distance": "extend.distance"})),
+        (
+            "surface_extend_to_plane",
+            json!({"type": "surface_extend", "body": "B", "edges": [o], "distance": "extend.distance", "mode": "curvature", "up_to": {"plane": plane("extend")}}),
+        ),
+        (
+            "surface_extend_to_face",
+            json!({"type": "surface_extend", "body": "B", "edges": [o], "distance": "extend.distance", "up_to": {"face": {"body": "B", "point": o}}, "neighbours": false}),
+        ),
         (
             "surface_ruled",
             json!({"type": "surface_ruled", "body": "B", "edges": [o], "distance": "ruled.distance", "mode": "angle", "angle": "ruled.angle",