fix(bedit): optimize block editor close and preserve warm render caches

Avoid rebuilding derived caches and clearing tessellation memos when closing the block editor on clean exit or discard. Scope entity delta computation to block record members instead of scanning and cloning the entire drawing, and add a fast path to the dirty check when no local commands were executed.

src/app/block_editor_tests.rs

@@ -1333,3 +1333,25 @@ fn locked_bedit_with_preselected_insert_opens_directly() {
         "EDIT_ME"
     );
 }
+
+#[test]
+fn locked_bedit_clean_close_preserves_block_epoch_and_render_caches() {
+    let (mut app, reference) = fixture();
+    let tab = app.active_tab;
+    let epoch_before = app.tabs[tab].scene.block_epoch;
+    begin(&mut app, reference);
+    let _ = app.dispatch_command("BCLOSE");
+    assert_eq!(app.tabs[tab].scene.block_epoch, epoch_before);
+}
+
+#[test]
+fn locked_bedit_discard_preserves_block_epoch_and_render_caches() {
+    let (mut app, reference) = fixture();
+    let tab = app.active_tab;
+    let epoch_before = app.tabs[tab].scene.block_epoch;
+    begin(&mut app, reference);
+    circle(&mut app, 1.0);
+    let _ = app.dispatch_command("BCLOSE");
+    let _ = app.update(Message::BlockEditChangesDiscard);
+    assert_eq!(app.tabs[tab].scene.block_epoch, epoch_before);
+}