feat(block-editor): add Save Block As

Create a new named definition from current session edits, restore the source checkpoint, and continue editing the copy with fresh local history. Add the name dialog and Save dropdown, clone supported owned metadata onto independent handles, and refuse unsupported relationships atomically.

src/app/command_driver/mod.rs

@@ -95,7 +95,8 @@ mod utilities;
 mod viewport;
 
 pub(crate) use clipboard::{
-    copy_to_clipboard_kernel, paste_entities_kernel, remap_ext_subtree_reference, PasteSpace,
+    copy_to_clipboard_kernel, paste_entities_kernel, remap_ext_subtree_reference, remap_object,
+    PasteSpace,
 };
 
 impl OpenCADStudio {