feat(grip): complete standard grip-shape vocabulary

Existing entity grip definitions already followed the conventional
CAD layout: square at vertices / endpoints, diamond at curve
centres / midpoints, triangle for directional control. Round out
the supported shape vocabulary with the two missing standards so
future entities (polyline straight-segment stretch grips, dimension
parameter grips, …) can pick the right marker without inventing
ad-hoc visuals:

- `GripShape::Rectangle` — direction-aware mid-segment stretch
  handle (wider-than-tall box).
- `GripShape::Circle` — parametric control (radius / dimension
  value).

New `rectangle_grip` / `circle_grip` helpers in `entities::common`
mirror the existing `square_grip` / `diamond_grip` / `triangle_grip`
constructors. `SelectionCanvas::draw` renders the two new variants;
the previously-used Square / Diamond / Triangle paths are
unchanged. Both new variants carry `#[allow(dead_code)]` until an
entity adopts them.

src/entities/common.rs

@@ -148,6 +148,26 @@ pub fn triangle_grip(id: usize, world: Vec3) -> GripDef {
     }
 }
 
+#[allow(dead_code)]
+pub fn rectangle_grip(id: usize, world: Vec3) -> GripDef {
+    GripDef {
+        id,
+        world,
+        is_midpoint: true,
+        shape: GripShape::Rectangle,
+    }
+}
+
+#[allow(dead_code)]
+pub fn circle_grip(id: usize, world: Vec3) -> GripDef {
+    GripDef {
+        id,
+        world,
+        is_midpoint: false,
+        shape: GripShape::Circle,
+    }
+}
+
 pub fn edit_prop(label: &'static str, field: &'static str, value: f64) -> Property {
     Property {
         label: label.into(),