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/blocks_palette.rs

@@ -427,7 +427,11 @@ impl OpenCADStudio {
         for def in deps.blocks {
             let scene = &mut self.tabs[i].scene;
             if redefine && def.name.eq_ignore_ascii_case(name) {
-                scene.redefine_block_raw(&def.name, def.base_point, def.entities);
+                if let Err(err) = scene.redefine_block_raw(&def.name, def.base_point, def.entities)
+                {
+                    self.command_line.push_error(&err);
+                    return Err(err);
+                }
             } else if scene.document.block_records.get(&def.name).is_none() {
                 scene.define_block_raw(&def.name, def.base_point, def.entities);
             }