Feat: Solid entity (2D) — render, grips, properties, transform

TruckConvertible renders the 4 perimeter edges as wireframe outline.
The scene injects a solid-fill HatchModel for the filled quad using the
existing hatch GPU pipeline (DXF Z-order: p0→p1→p3→p2).
Grippable provides 4 square corner grips; grip moves rebuild the fill model.
PropertyEditable exposes all 4 corners and thickness. Transformable handles
move/rotate/scale/mirror via the standard entity transform path.

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

src/entities/mod.rs

@@ -17,6 +17,7 @@ pub mod point;
 pub mod polyline;
 pub mod raster_image;
 pub mod ray;
+pub mod solid;
 pub mod spline;
 pub mod text;
 mod text_support;