Thin Extrude: walls of a set thickness along sketch curves (#721) (#991)

Thin Extrude builds walls of a set thickness along sketch curves, open chains and closed loops, with the wall centred, inside, outside or on both sides at two thicknesses. Saved under format 20 (thin on an Extrude); an extrude without thin saves as before. Closes #721.

crates/doc/src/format.rs

@@ -138,13 +138,18 @@
 //!   Under the same version, a fillet's style can be `curvature` (a G2 quintic blend; `radius`
 //!   is the equivalent circular setback). Fillets of the other styles save as before; 0.9.0
 //!   would refuse the unknown style.
+//! - 20: an Extrude can be thin (`thin`: `curves`, `thickness`, optional `side` and
+//!   `thickness2`): a wall along sketch curves instead of its profiles. Version 19 designs need
+//!   no transformation, and an extrude without `thin` saves as before; the version stamp makes
+//!   0.10.0 (which reads up to 19) ask for an update instead of ignoring the wall and filling
+//!   the sketch's profiles.
 
 use serde_json::Value;
 
 use crate::{DocError, Document, Result};
 
 /// The format this build writes.
-pub const FORMAT: u32 = 19;
+pub const FORMAT: u32 = 20;
 
 pub fn format_string(n: u32) -> String {
     format!("solidcraft/{n}")
@@ -161,7 +166,7 @@ pub fn version_of(s: &str) -> Option<u32> {
 
 /// JSON-level upgrades: entry `i` turns format `i + 1` into `i + 2`.
 type JsonStep = fn(&mut Value) -> Result<()>;
-const JSON_STEPS: [JsonStep; 18] = [
+const JSON_STEPS: [JsonStep; 19] = [
     |_| Ok(()),
     v2_to_v3,
     v3_to_v4,
@@ -180,6 +185,7 @@ const JSON_STEPS: [JsonStep; 18] = [
     |_| Ok(()),
     |_| Ok(()),
     |_| Ok(()),
+    |_| Ok(()),
 ];
 
 /// Stored face and edge names were worked out from geometry before format 14; they get the
@@ -1048,6 +1054,7 @@ mod tests {
         document
             .add_feature(
                 FeatureKind::Extrude {
+                    thin: None,
                     sketch,
                     profiles: ProfileSel::All,
                     extent: Extent {