Class-A surfacing 3/3: blend curves, curved-edge fillets, the G2 corner and continuity analysis (#1642)

Thanks @midasdf.

crates/doc/src/format/pending/fillet-corner-and-loft-continuity.rs

@@ -0,0 +1,7 @@
+//! The Class-A surface pass also lets a `Fillet` carry `corner` (`rolling`, the default and what
+//! every older design meant, or `g2`: a patch repeating each blend's surface up to the second
+//! order) and a `SurfaceLoft` carry `continuity` and `crown` and body edge-chain `sections` (a
+//! section picked as a `body` with `edges` instead of a `sketch`). A design without them saves as
+//! it always did — a loft without `continuity` reads and writes as `[g0, g0]`, the plain loft it
+//! always was — so older designs need no transformation; 0.13.x asks for an update instead of
+//! meeting a feature it can't read or dropping a corner style it does not round.