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/format.rs

@@ -212,6 +212,11 @@
 //!   parentheses (`2,5 mm` is `2.5 mm`, #1347; inside a function's parentheses the comma still
 //!   separates its arguments). Nothing new is saved, but an expression typed that way is saved as
 //!   typed; 0.12.0 asks for an update instead of failing to evaluate it.
+//!   Also under 22, a design can hold a `SurfaceBlend` (Blend Surface: `from`/`to` edge chains of
+//!   two bodies, optional `continuity`, `bulge`, `align`, `stitch`), and a `Patch` can carry a
+//!   `crown` (0 by default, and then not saved: a bump that vanishes on the loop, so the face
+//!   still meets the picked edges). Version 21 designs need no transformation; 0.12.0 asks for an
+//!   update instead of meeting a feature it can't read or dropping a patch's crown.
 
 use serde_json::Value;