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/block_ops_tests.rs

@@ -1519,3 +1519,47 @@ fn locked_attedit_attribute_flags_cases() {
     ));
     assert_eq!(preset_entity.get_value(), "edited_preset");
 }
+
+#[test]
+fn locked_bedit_insert_refuses_cycle() {
+    let mut app = app();
+    let name_a = "BEDIT_CYC_A";
+    let name_b = "BEDIT_CYC_B";
+
+    define_block(&mut app, name_a);
+
+    // B contains an insert of A
+    let ins_a = Insert::new(name_a.to_string(), Vector3::ZERO);
+    let i = tab(&app);
+    app.tabs[i]
+        .scene
+        .define_block_raw(name_b, Vector3::ZERO, vec![EntityType::Insert(ins_a)]);
+
+    // Now open BEDIT for block A
+    let insert_a = add_insert(&mut app, name_a);
+    begin_bedit(&mut app, insert_a);
+    assert!(bedit_open(&app));
+
+    // Try committing an INSERT of B into A inside BEDIT space:
+    // Inserting B into A would create A -> B -> A cycle
+    let _ = app.dispatch_command("-INSERT");
+    let mut feed = |text: &str| {
+        let _ = app.update(crate::app::Message::CommandInput(text.to_string()));
+        let _ = app.update(crate::app::Message::CommandSubmit);
+    };
+    feed(name_b);
+    feed("0,0");
+    feed("1");
+    feed("1");
+    feed("0");
+
+    // Verify refusal error on command line
+    let errs = errors(&app);
+    assert!(
+        errs.iter()
+            .any(|e| e.contains("circular dependency detected")),
+        "expected cycle error in errors: {errs:?}"
+    );
+
+    let _ = app.dispatch_command("BEDIT_DISCARD");
+}

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);
             }

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