feat: XREF management — auto-resolve, XATTACH, XREF, XRELOAD

- src/io/xref.rs: resolve_xrefs() scans block_records for is_xref flags,
  resolves paths relative to the opened file's directory, loads the
  referenced DWG/DXF and merges model-space entities into the xref block
- src/modules/insert/xattach.rs: XATTACH command with file-picker flow
  (XAttachPick/XAttachPickResult messages), prepare_xref_block() creates
  BlockRecord + Block entities and immediately resolves content
- XREF command: lists all external references in the drawing
- XRELOAD command: re-resolves all xref paths and rebuilds scene caches
- Auto-resolve on FileOpened: reports Loaded/NotFound per xref block
- CadCommand trait: added optional xattach_path() method
- ROADMAP.md: 1.9 XREF management marked ✅

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

src/app/cmd_result.rs

@@ -65,6 +65,27 @@ impl H7CAD {
                 self.refresh_properties();
             }
             CmdResult::CommitAndExit(entity) => {
+                // For XATTACH: ensure the xref block definition exists before
+                // committing the INSERT entity that references it.
+                // Extract path early to avoid borrow conflicts.
+                let xattach_path: Option<String> = {
+                    let tab = &self.tabs[i];
+                    if let Some(cmd) = tab.active_cmd.as_ref() {
+                        if cmd.name() == "XATTACH" {
+                            cmd.xattach_path()
+                        } else {
+                            None
+                        }
+                    } else {
+                        None
+                    }
+                };
+                if let Some(path) = xattach_path {
+                    crate::modules::insert::xattach::prepare_xref_block(
+                        &mut self.tabs[i].scene,
+                        &path,
+                    );
+                }
                 let label = self.history_label_from_active_cmd(i, "ENTITY");
                 self.push_undo_snapshot(i, label);
                 self.commit_entity(entity);