feat(props): editable block Name row + collapsible XYZ groups

- Block reference Name (Misc) is editable: the dropdown re-points the
  insert to another definition; typing a new name renames the definition
  everywhere (record re-key, Block marker, every INSERT reference).
  Anonymous (*) and xref blocks stay read-only, with the same guards
  added to Scene::rename_block.
- Custom single-control dropdown (text field + caret) built on the
  shared floating_below overlay so the list always opens downward,
  unlike iced's space-based menu placement.
- Consecutive "<Base> X/Y/Z" rows (Position, Scale, Start, Center, ...)
  collapse into one expandable summary row across all entities; the
  expand state persists across rebuilds and selection changes.
- The BEDIT block tab no longer offers the layout Rename/Delete
  context menu.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/ui/color_select.rs

@@ -295,6 +295,16 @@ pub fn color_grid_window(on_pick: impl Fn(AcadColor) -> Message) -> Element<'sta
     .into()
 }
 
+/// Render `base` inline with `popup` floating just below it — the shared
+/// dropdown mechanic for the panel's custom dropdowns (colour picker, block
+/// Name). Unlike iced's menu overlay it always opens downward.
+pub fn floating_below<'a>(
+    base: Element<'a, Message>,
+    popup: Element<'a, Message>,
+) -> Element<'a, Message> {
+    Element::new(Floating { base, popup })
+}
+
 /// A widget that renders `base` inline and `popup` as a floating overlay
 /// anchored just below it.
 struct Floating<'a> {