feat(block-editor): add Test Block preview

Preview the current unsaved block in a disposable drawing with independent history. Keep normal Draw tools available, append Close Test Block, and return to the original editor without saving. Guard persistence paths and handle refresh and parent-close lifecycle by stable document identity.

src/app/update/mod.rs

@@ -346,7 +346,17 @@ impl OpenCADStudio {
             .tabs
             .get(self.active_tab)
             .is_some_and(|tab| tab.active_block_edit.is_some());
-        self.ribbon.set_block_editing(block_editing);
+        let testing = self
+            .tabs
+            .get(self.active_tab)
+            .is_some_and(|tab| tab.block_test.is_some());
+        if testing {
+            self.ribbon.set_block_editing(false);
+            self.ribbon.set_block_testing(true);
+        } else {
+            self.ribbon.set_block_testing(false);
+            self.ribbon.set_block_editing(block_editing);
+        }
         task
     }
 
@@ -595,7 +605,11 @@ impl OpenCADStudio {
             }
         }
         #[cfg(target_arch = "wasm32")]
-        crate::sys::set_unsaved_changes_warning(self.tabs.iter().any(|tab| tab.dirty));
+        crate::sys::set_unsaved_changes_warning(
+            self.tabs
+                .iter()
+                .any(|tab| tab.dirty && tab.block_test.is_none()),
+        );
         if self.tabs[i].active_cmd.is_none() {
             self.block_palette.placing = None;
         }