Class-A surfacing 1/3: the continuity core, a G3 free-form blend and patch crown (#1249)

The continuity core (kernel bridge: Hermite control rows reproducing a support surface's position, tangent, curvature and change of curvature, G0 to G3), surface.blend between two edge chains (#764), and a crown for fitted patches. Existing patches and fillets are unchanged by default (crown 0 is the plain patch bit for bit). SurfaceBlend and Patch.crown are noted under format 22.

crates/doc/src/feature_walk_tests/golden_exprs.txt

@@ -328,6 +328,7 @@ surface_sweep_options
     Scale = surface_sweep.scale (Unitless)
   expressions: surface_sweep.twist, surface_sweep.scale
 surface_loft
+surface_blend
 surface_offset
   inputs:
     Distance = soff.distance (Length)