fix: route iXex collision zones through m_unselected for effective exclusion

params.excluded_regions flows into libnest2d PlacementConfig.m_excluded_regions
which is declared but never consumed by the NFP placer — the field is dead code.
m_unselected is the working path: it becomes fixeditems that are preloaded as
physical fixed obstacles in the NFP computation, same as the wipe tower.

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

src/slic3r/GUI/Jobs/ArrangeJob.cpp

@@ -566,7 +566,7 @@ void ArrangeJob::process(Ctl &ctl)
                 ap.bed_idx       = current_plate_index;
                 ap.height        = 1;
                 ap.name          = "IXexCollisionZone";
-                params.excluded_regions.emplace_back(std::move(ap));
+                m_unselected.emplace_back(std::move(ap));
             }
         }
     }