feat(properties): show entity handle in the General section

Adds a read-only Handle row at the top of the properties palette so an
entity can be identified by handle (e.g. when cross-referencing a file
dump or reporting a specific object).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/cache/properties.rs

@@ -14,6 +14,11 @@ pub fn general_section(entity: &EntityType) -> PropSection {
     PropSection {
         title: "General".into(),
         props: vec![
+            Property {
+                label: "Handle".into(),
+                field: "handle",
+                value: PropValue::ReadOnly(common.handle.value().to_string()),
+            },
             Property {
                 label: "Layer".into(),
                 field: "layer",