feat: WBLOCK — write block or selected entities to external file

- src/modules/insert/wblock.rs: extract_block_to_doc() copies a named
  block's entities into a fresh CadDocument; extract_entities_to_doc()
  does the same for a selection set; layer definitions are carried over
- WBLOCK <name> writes the named block; WBLOCK * writes selected entities
- WblockSave / WblockSaveResult messages drive the async save-dialog flow
- ROADMAP.md: 1.10 WBLOCK marked ✅

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/modules/insert/mod.rs

@@ -7,6 +7,7 @@ mod cylinder;
 pub(crate) mod insert_block;
 mod open_obj;
 mod sphere;
+pub(crate) mod wblock;
 pub(crate) mod xattach;
 
 use crate::modules::{CadModule, RibbonGroup};
@@ -40,6 +41,7 @@ impl CadModule for InsertModule {
                 tools: vec![
                     create_block::tool().into(),
                     insert_block::tool().into(),
+                    wblock::tool().into(),
                     xattach::tool().into(),
                 ],
             },