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/scene/view/dispatch.rs

@@ -17,11 +17,14 @@ pub fn properties_sectioned(
     entity: &EntityType,
     text_style_names: &[String],
 ) -> Vec<PropSection> {
-    let general = properties::general_section(entity);
-    let geometry = entity
-        .geometry_properties(text_style_names)
-        .unwrap_or_else(|| properties::fallback_properties(handle, entity));
-    vec![general, geometry]
+    let mut sections = vec![properties::general_section(entity)];
+    let groups = entity.geometry_properties(text_style_names);
+    if groups.is_empty() {
+        sections.push(properties::fallback_properties(handle, entity));
+    } else {
+        sections.extend(groups);
+    }
+    sections
 }
 
 pub fn apply_common_prop(entity: &mut EntityType, field: &str, value: &str) {