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/mod.rs

@@ -412,6 +412,11 @@ pub enum Message {
     ImagePick,
     /// Result of the image file picker + pixel dimension decode.
     ImagePickResult(Result<(std::path::PathBuf, u32, u32), String>),
+    // ── XREF ──────────────────────────────────────────────────────────────
+    /// Open file-picker dialog for XATTACH command (async).
+    XAttachPick,
+    /// Result of the XATTACH file picker.
+    XAttachPickResult(Result<std::path::PathBuf, String>),
 }
 
 impl H7CAD {