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 <[email protected]>

src/entities/point.rs

@@ -178,15 +178,15 @@ fn grips(pt: &Point) -> Vec<GripDef> {
     vec![square_grip(0, p)]
 }
 
-fn properties(pt: &Point) -> PropSection {
-    PropSection {
+fn properties(pt: &Point) -> Vec<PropSection> {
+    vec![PropSection {
         title: "Geometry".into(),
         props: vec![
             edit("X", "loc_x", pt.location.x),
             edit("Y", "loc_y", pt.location.y),
             edit("Z", "loc_z", pt.location.z),
         ],
-    }
+    }]
 }
 
 fn apply_geom_prop(pt: &mut Point, field: &str, value: &str) {