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 <[email protected]>

src/scene/mod.rs

@@ -204,6 +204,20 @@ pub fn vp_effective_scale(custom_scale: f64, view_height: f64, vp_height: f64) -
     1.0
 }
 
+/// A block name a user may assign: non-empty, no control characters, none of
+/// the symbol-table-reserved characters (`*` marks anonymous blocks, `|`
+/// xref-dependent ones; the rest are the DXF symbol-name exclusions).
+pub(crate) fn valid_block_name(name: &str) -> bool {
+    !name.is_empty()
+        && !name.chars().any(|c| {
+            c.is_control()
+                || matches!(
+                    c,
+                    '<' | '>' | '/' | '\\' | '"' | ':' | ';' | '?' | '*' | '|' | ',' | '=' | '`'
+                )
+        })
+}
+
 /// Pre-built entity caches returned by [`build_derived_caches`].
 /// Produced in the file-load background task so the UI thread only assigns.
 #[derive(Debug, Clone)]