Replace placement calls with region nodes

tests/trace_unsettled.rs

@@ -62,8 +62,8 @@ fn dump(label: &str, report: &diag::Report, text: WidgetId) {
             TraceEvent::SizeRead { id, reader, size } if *id == text => {
                 println!("  size read by {reader:?}: {size}")
             }
-            TraceEvent::Placed { id, parent, region } if *id == text => {
-                println!("  placed by {parent:?} at {region:?}")
+            TraceEvent::RegionNode { id, parent, region } if *id == text => {
+                println!("  region node under {parent:?} at {region:?}")
             }
             TraceEvent::Reuse { id, outcome } if *id == text => println!("  reuse: {outcome:?}"),
             _ => {}