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 <[email protected]>

src/app/mod.rs

@@ -417,6 +417,11 @@ pub enum Message {
     XAttachPick,
     /// Result of the XATTACH file picker.
     XAttachPickResult(Result<std::path::PathBuf, String>),
+    // ── WBLOCK ────────────────────────────────────────────────────────────
+    /// Trigger the WBLOCK save dialog for `block_name` (or `*` = selection).
+    WblockSave(String),
+    /// Result of the WBLOCK save path dialog.
+    WblockSaveResult(String, Option<std::path::PathBuf>),
 }
 
 impl H7CAD {