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/commands/fileops.rs

@@ -38,7 +38,7 @@ impl OpenCADStudio {
                     let mut saved = 0usize;
                     let mut skipped = 0usize;
                     for t in 0..self.tabs.len() {
-                        if self.tabs[t].is_start {
+                        if self.tabs[t].is_start || self.tabs[t].block_test.is_some() {
                             continue;
                         }
                         if let Some(path) = self.tabs[t].current_path.clone() {