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/lwpolyline.rs

@@ -322,9 +322,9 @@ fn grips(pline: &LwPolyline) -> Vec<GripDef> {
     out
 }
 
-fn properties(pline: &LwPolyline) -> PropSection {
+fn properties(pline: &LwPolyline) -> Vec<PropSection> {
     let (length, area) = length_and_area(pline);
-    PropSection {
+    vec![PropSection {
         title: "Geometry".into(),
         props: vec![
             ro("Vertices", "vertices", pline.vertices.len().to_string()),
@@ -337,7 +337,7 @@ fn properties(pline: &LwPolyline) -> PropSection {
             ro("Length", "length", format!("{length:.4}")),
             ro("Area", "area", format!("{area:.4}")),
         ],
-    }
+    }]
 }
 
 /// Path length and enclosed area of a polyline, accounting for arc
@@ -587,7 +587,7 @@ impl crate::entities::traits::PropertyEditable for LwPolyline {
     fn geometry_properties(
         &self,
         _text_style_names: &[String],
-    ) -> crate::scene::model::object::PropSection {
+    ) -> Vec<crate::scene::model::object::PropSection> {
         properties(self)
     }
     fn apply_geom_prop(&mut self, field: &str, value: &str) {