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/modules/insert/mod.rs

@@ -7,6 +7,7 @@ mod cylinder;
 pub(crate) mod insert_block;
 mod open_obj;
 mod sphere;
+pub(crate) mod xattach;
 
 use crate::modules::{CadModule, RibbonGroup};
 
@@ -36,7 +37,11 @@ impl CadModule for InsertModule {
             },
             RibbonGroup {
                 title: "Block",
-                tools: vec![create_block::tool().into(), insert_block::tool().into()],
+                tools: vec![
+                    create_block::tool().into(),
+                    insert_block::tool().into(),
+                    xattach::tool().into(),
+                ],
             },
         ]
     }