fix: make viewports from external DXF/DWG files visible and enterable

Two root causes prevented paper-space viewports from loading correctly:

1. acadrust's ViewportStatusFlags::from_bits() maps bit 0 → is_on, but
   the real DXF/DWG spec uses bit 15 (0x8000) for "viewport on". Post-load
   fixup in io/mod.rs now corrects this for both DXF and DWG files: when
   bit 0 is clear but bit 15 is set, is_on is forced true and locked is
   derived from bit 14 (0x4000).

2. Some DXF exporters write id=-1 for all viewport entities instead of the
   standard id=1 (sheet) / id≥2 (user) convention. The is_user_viewport_id()
   helper now treats any id ≠ 0 and ≠ 1 as a user viewport, covering both
   the standard id≥2 and the non-standard id=-1 cases.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/update.rs

@@ -1696,7 +1696,7 @@ impl H7CAD {
                                     if let Some(AcadEntityType::Viewport(vp)) =
                                         self.tabs[i].scene.document.get_entity(h)
                                     {
-                                        if vp.id > 1 { Some(h) } else { None }
+                                        if Scene::is_user_viewport_id(vp.id) { Some(h) } else { None }
                                     } else {
                                         None
                                     }