feat: Shape entity support (render + grips + props + transform)

- src/entities/shape.rs: diamond marker at insertion point (SHX glyphs
  cannot be rendered without the binary .shx file, so a placeholder is used)
- Grips: single insertion-point grip
- Properties: Name, Style (read-only), Insert X/Y/Z, Size, Rotation
- Transformable: translate, mirror, scale, rotate
- Registered in entities/mod.rs, entities/traits.rs, app/commands.rs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/entities/mod.rs

@@ -18,6 +18,7 @@ pub mod polyline;
 pub mod raster_image;
 pub mod underlay;
 pub mod ray;
+pub mod shape;
 pub mod solid;
 pub mod spline;
 pub mod text;