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>

ROADMAP.md

@@ -18,7 +18,7 @@ Durum simgeleri: ✅ Tamamlandı · 🔧 Kısmen yapıldı · ⬜ Yapılmadı
 | 1.6 | Undo / Redo (snapshot stack) | ✅ |
 | 1.7 | PDF dışa aktarma (CTB/STB plot style) | ✅ |
 | 1.8 | Fiziksel yazıcıya yazdırma | ⬜ |
-| 1.9 | XREF (dış referans) yönetimi | ⬜ |
+| 1.9 | XREF (dış referans) yönetimi | ✅ Auto-resolve on open, XATTACH/XREF/XRELOAD commands |
 | 1.10 | WBLOCK — bloğu dış dosyaya yazma | ⬜ |
 | 1.11 | Serde entegrasyonu (JSON/alternatif I/O) | ⬜ |
 | 1.12 | Bozuk DWG kurtarma (failsafe parse) | ⬜ |