Sketch: associative, editable sketch patterns (#1103)

Rectangular and circular sketch patterns stay linked to their source: editing the pattern's count, spacing or the source geometry updates the copies, and the copies keep their point and radius relations. Patterns are saved under format 20; older designs keep their independent copies.

crates/engine/src/cmd/sketch_constraints.rs

@@ -431,6 +431,8 @@ fn anchor(sk: &Sketch, k: &ConstraintKind) -> Option<Vec2> {
     let pt = |p: usize| sk.point(p);
     match *k {
         OffsetDistance { ref output, .. } => sk.offset_dimension_points(output).map(|(a, b)| (a + b) * 0.5),
+        PatternPoint { q, center, .. } => pt(center.unwrap_or(q)),
+        PatternRadius { b, .. } => mid(b),
         Coincident { p, .. } | Fix { p } | Midpoint { p, .. } => pt(p),
         PointOnCurve { p, .. } => pt(p),
         HorizontalPoints { p, q } | VerticalPoints { p, q } | Symmetric { p, q, .. } => Some((pt(p)? + pt(q)?) * 0.5),