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/underlay.rs

@@ -146,13 +146,13 @@ impl Grippable for Underlay {
 // ── PropertyEditable ──────────────────────────────────────────────────────────
 
 impl PropertyEditable for Underlay {
-    fn geometry_properties(&self, _text_style_names: &[String]) -> PropSection {
+    fn geometry_properties(&self, _text_style_names: &[String]) -> Vec<PropSection> {
         let type_str = match self.underlay_type {
             acadrust::entities::UnderlayType::Pdf => "PDF",
             acadrust::entities::UnderlayType::Dwf => "DWF",
             acadrust::entities::UnderlayType::Dgn => "DGN",
         };
-        PropSection {
+        vec![PropSection {
             title: "Geometry".into(),
             props: vec![
                 ro("Type", "ul_type", type_str),
@@ -198,7 +198,7 @@ impl PropertyEditable for Underlay {
                     if self.clip_inverted { "Yes" } else { "No" },
                 ),
             ],
-        }
+        }]
     }
 
     fn apply_geom_prop(&mut self, field: &str, value: &str) {