refactor(properties): entity property groups return Vec<PropSection>

Groundwork for making the Properties panel conform to the per-entity group/row
spec (PROPERTIES.md). The panel already renders a Vec of sections, but each
entity could only express a single geometry section.

Change PropertyEditable::geometry_properties (and the EntityTypeOps enum
dispatch) to return Vec<PropSection>, update the two impl macros, and flatten
in properties_sectioned so the panel shows [General] + the entity's groups.
Every entity builder is wrapped to return its existing section as a one-element
Vec — pure mechanical change, no rows or titles altered, panel output identical.

Enables Phase 2: rewriting each entity's groups/rows to match the spec.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/entities/circle.rs

@@ -118,8 +118,8 @@ fn grips(circle: &Circle) -> Vec<GripDef> {
     ]
 }
 
-fn properties(circle: &Circle) -> PropSection {
-    PropSection {
+fn properties(circle: &Circle) -> Vec<PropSection> {
+    vec![PropSection {
         title: "Geometry".into(),
         props: vec![
             edit("Center X", "center_x", circle.center.x),
@@ -137,7 +137,7 @@ fn properties(circle: &Circle) -> PropSection {
                 format!("{:.4}", circle.radius * 2.0 * std::f64::consts::PI),
             ),
         ],
-    }
+    }]
 }
 
 fn apply_geom_prop(circle: &mut Circle, field: &str, value: &str) {