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/control/actions.rs

@@ -410,6 +410,12 @@ impl OpenCADStudio {
     /// this needs no undo snapshot. Format follows the path extension.
     #[cfg(not(target_arch = "wasm32"))]
     pub(super) fn control_wblock(&mut self, req: &Value) -> Result<Task<Message>, Value> {
+        if self.tabs[self.active_tab].block_test.is_some() {
+            return Err(failure(
+                "temporary_document",
+                "Test Block is temporary and cannot be saved",
+            ));
+        }
         let path = string(req, "path")?;
         let document = &self.tabs[self.active_tab].scene.document;
         // A template base keeps its tables/styles in the exported file

src/app/control/mod.rs

@@ -1148,6 +1148,12 @@ impl OpenCADStudio {
             "view_focus" => self.control_view_focus(req)?,
             #[cfg(not(target_arch = "wasm32"))]
             "save" => {
+                if self.tabs[i].block_test.is_some() {
+                    return Err(failure(
+                        "temporary_document",
+                        "Test Block is temporary and cannot be saved",
+                    ));
+                }
                 let path = req["path"]
                     .as_str()
                     .map(std::path::PathBuf::from)
@@ -1198,6 +1204,12 @@ impl OpenCADStudio {
             // Web: no filesystem path; the drawing comes back in the reply.
             #[cfg(target_arch = "wasm32")]
             "save" => {
+                if self.tabs[i].block_test.is_some() {
+                    return Err(failure(
+                        "temporary_document",
+                        "Test Block is temporary and cannot be saved",
+                    ));
+                }
                 use base64::Engine as _;
                 let default_is_dxf = crate::io::source_is_dxf(None, &self.tabs[i].scene.document);
                 let (version, is_dxf) = super::automation::requested_save_target(

src/app/control/sheets.rs

@@ -20,7 +20,10 @@ impl OpenCADStudio {
             self.set_control_result(json!({ "closed": false, "reason": "start_page" }));
             return Ok(Task::none());
         }
-        if self.tabs[index].dirty && !req["discard"].as_bool().unwrap_or(false) {
+        if self.tabs[index].dirty
+            && self.tabs[index].block_test.is_none()
+            && !req["discard"].as_bool().unwrap_or(false)
+        {
             return Err(failure(
                 "document_dirty",
                 "Save the document or pass discard:true",