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/mod.rs

@@ -9,6 +9,9 @@ pub(crate) mod config;
 pub use automation::{export_headless, serve};
 mod annotation_data;
 mod block_editor;
+mod block_test;
+#[cfg(test)]
+mod block_test_tests;
 mod block_save_as;
 #[cfg(test)]
 mod block_editor_tests;