Fix #511: Live Corners on any path's corners (stars, polygons, pen paths)

Corner widgets only ever showed on live rectangles: CornerWidgets::of bailed
out unless the selection's live shape was LiveShape::Rectangle, and
object.setLiveShape only edited rectangle radii. A star is a plain path, so it
had no widgets with either selection tool.

- geom::corners: the corners of any path (anchors without handles between two
  straight sides) and the path with them cut round, inverted round or
  chamfered, each corner a circle tangent to both sides, its radius no larger
  than takes the cut halfway along its shorter side. Rectangles now cut their
  corners with it (bit-identical anchors and layout).
- doc::corners::LiveCorners/set_corners: one model for a live rectangle's,
  a live polygon's (new radii/kinds) and any other path's corners (new
  LiveShape::Path keeping the uncut outline, plain again once nothing is cut);
  radii scale with transforms (Scale Corners) and stay circular.
- object.setLiveShape takes anchor indices for `corners` on any path; polygons
  keep a shared radius as their sides change.
- Widgets: Direct Selection on any path, Selection on live rectangles and
  polygons; placed on each corner's bisector, the drag follows it, the red
  limit outline marks corners at their own limit; the angle preference and
  View > Hide Corner Widget apply. Corners dialog and the Properties and
  Transform panels' Corner Radius (now on polygons too) work on them.
- Tests for the star, the polygon and a pen path in geom, doc, tools, engine
  and ui-egui; docs and ROADMAP.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/control-protocol.md

@@ -291,9 +291,9 @@ Width Point Edit: double-clicking a width point with the Width tool, or `ui.widt
 value: true}` then confirm (the Delete button) removes the point with `stroke.widthPoint.remove`.
 
 Corners: double-clicking a Live Corners widget with the Selection or Direct Selection tool, or `ui.corners {id?,
-corners?}`, opens the `corners` dialog for a live rectangle's corners (the Direct-Selected ones, else all four; fields
-`id`, `corners`: indices 0–3 clockwise from the top-left, `kind`: round, invertedRound or chamfer, `radius` in points;
-`kind` or `radius` is absent while the corners differ). `ui.dialog.confirm` runs `object.setLiveShape` with them.
+corners?}`, opens the `corners` dialog for a path's corners (the Direct-Selected ones, else every corner; fields `id`,
+`corners`: anchor indices of the path with its corners uncut, a rectangle's 0–3 clockwise from the top-left, `kind`:
+round, invertedRound or chamfer, `radius` in points; `kind` or `radius` is absent while the corners differ). `ui.dialog.confirm` runs `object.setLiveShape` with them.
 
 Perspective plane options: double-clicking a plane widget of the perspective grid, or `ui.perspectivePlane {plane}`,
 opens the `perspectivePlane` dialog (fields `plane`: left, right or ground; `location`: points along the plane's