Fix #442: alike circular corners on rectangles that aren't square

Live rectangles drew elliptical corners after an uneven scale in v0.4.0;
inconsistent on a rectangle that isn't square:

- doc: LiveShape::fold_scale moves an uneven scale left in `xf` (files
  saved before #291) into the size and radii; `transform` uses it, and
  `folded()`/`corner_style` give Live Corners document units.
- engine: object.setLiveShape folds that scale first, so setting a radius
  or kind (fields, dialog, widgets, MCP) gives circular corners on such
  shapes instead of keeping the ellipses.
- geom: max_corner_radius/fitted_corner_radius are the one limit every
  corner shares (half the shorter side).
- tools: corner widgets sit at the centres of the corners as drawn (a
  radius past the limit put them off the arc, further in along the long
  side), drag in document units on stretched shapes, and outline the
  edited corners in red at the limit, as the reference app does.
- svg: CSS export writes the drawn radii (CSS would draw a radius past the
  limit when its neighbours leave room).
- docs: mcp.md Live Corners, ROADMAP.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/object.rs

@@ -1038,6 +1038,9 @@ fn set_live_shape(s: &mut Session, p: &Value) -> Result<Value> {
     s.edit("Live Shape", |d, sel| {
         for id in &ids {
             let Some(NodeKind::Path { path, live: Some(live), .. }) = d.node_mut(*id).map(|n| &mut n.kind) else { continue };
+            // Radii are document lengths and corners circular, also on a rectangle from a file
+            // that kept an uneven scale in its transform (#442).
+            live.fold_scale();
             let partial = sel.anchors.get_mut(id);
             let picked = corners.unwrap_or_else(|| live.picked_corners(partial.as_deref()));
             let (layout, selected) = (live.anchor_corners(), partial.as_deref().map(|a| live.corners_of(a)));