fix(blocks): prevent circular block nesting and cut render cycles

Introduce would_create_cycle in block_service to prevent recursive block definitions with case-insensitive matching across block records. Enforce cycle refusal at INSERT commit in BEDIT and during block redefinition (redefine_block_raw) with a shared error message. Ensure render-time recursion truncates safely with a one-time session warning on cyclic documents.

src/app/update/dialog.rs

@@ -580,11 +580,14 @@ impl OpenCADStudio {
                 .define_block_raw(&def.name, def.base_point, def.entities);
         }
         if redefine {
-            self.tabs[i].scene.redefine_block_raw(
+            if let Err(err) = self.tabs[i].scene.redefine_block_raw(
                 &name,
                 codec::types::Vector3::new(base.x, base.y, base.z),
                 entities,
-            );
+            ) {
+                self.command_line.push_error(&err);
+                return Err(err);
+            }
         } else {
             self.tabs[i]
                 .scene