Unstitch and Reverse Normal (#856)

Unstitch takes a body's faces out as surface bodies; Reverse Normal turns a surface body's faces over. Both are saved in format 18. Closes #766.

crates/doc/src/format.rs

@@ -104,13 +104,17 @@
 //!   Construction Geometries); a sketch that shows its construction geometry saves as before.
 //!   A `SurfaceExtrude` feature (Surface Extrude: `sketch`, optional `profiles`, `curves` and
 //!   `extent`, walls only) is first saved in 17; 0.8.0 would refuse the unknown feature.
+//! - 18: surface features 0.9.0 doesn't know: `Unstitch` (a body's faces taken out as surface
+//!   bodies) and `ReverseNormal` (a surface body's faces turned over), each a `body` and
+//!   optional `faces` points. Version 17 designs need no transformation; the version stamp makes
+//!   0.9.0 (which reads up to 17) ask for an update instead of meeting a feature it can't read.
 
 use serde_json::Value;
 
 use crate::{DocError, Document, Result};
 
 /// The format this build writes.
-pub const FORMAT: u32 = 17;
+pub const FORMAT: u32 = 18;
 
 pub fn format_string(n: u32) -> String {
     format!("solidcraft/{n}")
@@ -127,7 +131,7 @@ 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; 16] = [
+const JSON_STEPS: [JsonStep; 17] = [
     |_| Ok(()),
     v2_to_v3,
     v3_to_v4,
@@ -144,6 +148,7 @@ const JSON_STEPS: [JsonStep; 16] = [
     |_| Ok(()),
     |_| Ok(()),
     |_| Ok(()),
+    |_| Ok(()),
 ];
 
 /// Stored face and edge names were worked out from geometry before format 14; they get the