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/solid.rs

@@ -97,8 +97,8 @@ impl Grippable for Solid {
 }
 
 impl PropertyEditable for Solid {
-    fn geometry_properties(&self, _text_style_names: &[String]) -> PropSection {
-        PropSection {
+    fn geometry_properties(&self, _text_style_names: &[String]) -> Vec<PropSection> {
+        vec![PropSection {
             title: "Geometry".into(),
             props: vec![
                 edit("P1 X", "sl_p1x", self.first_corner.x),
@@ -115,7 +115,7 @@ impl PropertyEditable for Solid {
                 edit("P4 Z", "sl_p4z", self.fourth_corner.z),
                 edit("Thickness", "sl_thick", self.thickness),
             ],
-        }
+        }]
     }
 
     fn apply_geom_prop(&mut self, field: &str, value: &str) {