Implementation of external references (XREFs) for CAD drawings, including loading, display, path management, and associated commands/UI.

## Summary
External references are missing (`docs/gaps.md` F2, alpha blocker): XATTACH, the XREF palette, overlay, path handling, BIND, REFEDIT and XCLIP. #213 now keeps xref block paths through a DXF save, but the referenced drawings are never loaded or drawn.

## Scope (part 1 here; REFEDIT and XCLIP as follow-ups if they don't fit)
- **Xref model:** a block definition flagged as an xref (attach or overlay) with a path. The path is saved as given (full, relative or none) and resolved against the host drawing's folder, the saved path and a search path (PROJECTNAME/XREF search paths kept simple).
- **Loading:** a referenced DXF or DWG is read through the io read hook. Its model space becomes the block's content, and its layers, linetypes and styles are prefixed `xref|name`. Nesting is bounded (MAX_BLOCK_DEPTH); circular references are detected; missing files are reported as "Not Found" without breaking the host.
- **Commands:**
- XATTACH (path, Attach/Overlay, insertion/scale/rotation);
- XREF / -XREF (?/Bind/Detach/Path/Unload/Reload/Overlay/Attach);
- XBIND (bind individual named objects);
- BIND (Bind/Insert) turns an xref into a normal block.
- JSON forms; programmatic calls never open dialogs (no file picker from JSON).
- **External References palette:** a list with status (Loaded/Unloaded/Not Found/Unreferenced), type and path, plus Reload/Detach/Unload/Bind/Open. Kept thin.
- **DXF:** xref BLOCK flags (4 xref, 8 overlay, 32 resolved) and the path (group 1). X...