Extrude: a symmetric distance is the total thickness (#1260)

A symmetric solid extrude of 10 mm is now 10 mm thick, 5 mm each side of the sketch plane, and switching from One Side to Symmetric keeps the thickness entered. Under format 21 the extent records its convention; designs saved without it keep the per-side reading, so their geometry and parameters don't change. Fixes #1236.

Co-authored-by: Karl-VonRichter <Karl-VonRichter@users.noreply.github.com>

crates/doc/src/format.rs

@@ -179,6 +179,11 @@
 //!   neighbouring edges as they are (`neighbours: false`). An Extend without them builds the
 //!   planar extension it always did and saves as before; 0.11.0 asks for an update instead of
 //!   dropping them and extending a plane by the distance.
+//!   Under the same version, an extrude's extent records how a symmetric `distance` is read
+//!   (`convention`: `total_distance`, split evenly around the start plane, which new solid
+//!   extrudes save; or `legacy_per_side`). An extent without it keeps the per-side reading
+//!   designs always had, so their thickness and parameters don't change; 0.11.0 asks for an
+//!   update instead of doubling a total-distance extrude.
 
 use serde_json::Value;
 
@@ -1131,6 +1136,7 @@ mod tests {
                     sketch,
                     profiles: ProfileSel::All,
                     extent: Extent {
+                        convention: crate::ExtrudeConvention::LegacyPerSide,
                         distance: "10".into(),
                         direction: Direction::Negative,
                         distance2: None,