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/dimension.rs

@@ -57,7 +57,7 @@ fn base_props(base: &DimensionBase) -> Vec<crate::scene::model::object::Property
     ]
 }
 
-fn properties(dim: &Dimension) -> PropSection {
+fn properties(dim: &Dimension) -> Vec<PropSection> {
     let mut props = base_props(dim.base());
     match dim {
         Dimension::Aligned(d) => {
@@ -129,10 +129,10 @@ fn properties(dim: &Dimension) -> PropSection {
             ));
         }
     }
-    PropSection {
+    vec![PropSection {
         title: "Geometry".into(),
         props,
-    }
+    }]
 }
 
 fn linear_like_props(
@@ -579,7 +579,7 @@ fn mirror_point(p: &mut acadrust::types::Vector3, p1: DVec3, p2: DVec3) {
 }
 
 impl PropertyEditable for Dimension {
-    fn geometry_properties(&self, _text_style_names: &[String]) -> PropSection {
+    fn geometry_properties(&self, _text_style_names: &[String]) -> Vec<PropSection> {
         properties(self)
     }