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 <[email protected]>

src/app/commands.rs

@@ -3276,6 +3276,7 @@ fn entity_type_name(entity: &acadrust::EntityType) -> &'static str {
         acadrust::EntityType::AttributeEntity(_)    => "ATTRIB",
         acadrust::EntityType::Leader(_)             => "LEADER",
         acadrust::EntityType::Tolerance(_)          => "TOLERANCE",
+        acadrust::EntityType::Shape(_)              => "SHAPE",
         _ => "ENTITY",
     }
 }