Joints: gears, rack and pinion, and screws (#1524)

Motion links can carry a coupling (gear pair, rack and pinion, screw) whose ratio follows radii, picked round faces or pitch. Design format 23.

Co-authored-by: SychPL <SychPL@users.noreply.github.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/doc/src/format.rs

@@ -222,13 +222,20 @@
 //!   parameters); the source's `at`, `height` and `angle` then hold the numbers they last came
 //!   to. A text without `exprs` is set as it always was; 0.12.0 asks for an update instead of
 //!   dropping the expressions and keeping the text at its last numbers.
+//! - 23: a motion link can carry a `coupling`: a gear pair (`ra`, `rb`, `internal`), a rack and
+//!   pinion (`radius`) or a screw (`pitch`), each size a number in mm, an expression or a picked
+//!   round face, and `reverse`. The link's `ratio` is worked out from it whenever the design
+//!   changes and saved as well, so a link reads the same without it. Version 22 designs need no
+//!   transformation: a link without `coupling` is a plain motion link, as it always was; 0.13.0
+//!   (which reads up to 22) asks for an update instead of keeping a gear's ratio when its radius
+//!   changes.
 
 use serde_json::Value;
 
 use crate::{DocError, Document, Result};
 
 /// The format this build writes.
-pub const FORMAT: u32 = 22;
+pub const FORMAT: u32 = 23;
 
 pub fn format_string(n: u32) -> String {
     format!("solidcraft/{n}")
@@ -245,7 +252,7 @@ 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; 21] = [
+const JSON_STEPS: [JsonStep; 22] = [
     |_| Ok(()),
     v2_to_v3,
     v3_to_v4,
@@ -267,6 +274,7 @@ const JSON_STEPS: [JsonStep; 21] = [
     |_| Ok(()),
     |_| Ok(()),
     |_| Ok(()),
+    |_| Ok(()),
 ];
 
 /// Stored face and edge names were worked out from geometry before format 14; they get the