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/document.rs

@@ -207,6 +207,7 @@ pub(super) struct DocumentTab {
     pub(super) history: HistoryState,
     /// Drawing history is suspended while `history` belongs to BEDIT.
     pub(super) block_edit_history: Option<super::block_editor::BlockEditHistory>,
+    pub(super) block_test: Option<super::block_test::BlockTestSession>,
     pub(super) active_layer: String,
     /// Block references whose properties panel shows per-axis Scale X/Y/Z even
     /// though the three factors are currently equal — the user unchecked the
@@ -660,6 +661,7 @@ impl DocumentTab {
             dyn_active: 0,
             history: HistoryState::default(),
             block_edit_history: None,
+            block_test: None,
             active_layer: "0".to_string(),
             props_asym_scale: std::collections::HashSet::new(),
             graph: Default::default(),