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_offsets.rs

@@ -427,6 +427,7 @@ mod tests {
                         sketch: if missing_sketch { 999_999 } else { id },
                         profiles: ProfileSel::OffsetBoundaries { associations: vec!["deleted-o1".into()] },
                         extent: crate::Extent {
+                            convention: crate::ExtrudeConvention::LegacyPerSide,
                             distance: "2".into(),
                             direction: crate::Direction::Positive,
                             distance2: None,