Replace placement calls with region nodes

tests/layout_diagnostics.rs

@@ -30,7 +30,7 @@ fn a_selected_widget_retains_its_layout_events() {
     diagnostics::clear_traced_widgets();
     let _ = diagnostics::take();
     let mut harness = Harness::new((400, 200));
-    let leaf = rect(Color::RED).add(&mut harness.rsc);
+    let leaf = rect(Color::RED).region_node().add(&mut harness.rsc);
     let other = rect(Color::BLUE).add(&mut harness.rsc);
     let root = (leaf, other).span(Dir::RIGHT).add(&mut harness.rsc);
     harness.set_root(root);
@@ -46,7 +46,7 @@ fn a_selected_widget_retains_its_layout_events() {
         report
             .traces()
             .iter()
-            .any(|event| matches!(event, TraceEvent::Placed { id, .. } if *id == leaf.id()))
+            .any(|event| matches!(event, TraceEvent::RegionNode { id, .. } if *id == leaf.id()))
     );
     assert!(
         report