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/command/mod.rs

@@ -227,6 +227,12 @@ pub trait CadCommand: Send {
         false
     }
 
+    /// If this command is XATTACH, returns the file path to attach.
+    /// Default: None.
+    fn xattach_path(&self) -> Option<String> {
+        None
+    }
+
     /// Called instead of `on_point` when the command needs a tangent pick
     /// and the snap system found a tangent object.
     fn on_tangent_point(&mut self, obj: TangentObject, hit: Vec3) -> CmdResult {