Point clouds: scan and region visibility in the cloud record

The manager's switches write the cloud's hidden scans (by the scan
identifier its project and scan file carry) and hidden regions (the
unassigned points are region 0), as the reference stores them, instead
of extended data. A parent switch shows mixed while some of its rows
are on. A cloud that shows none of its points draws its extents box.
The codec reads and writes both lists.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/commands/point_cloud.rs

@@ -146,8 +146,8 @@ impl OpenCADStudio {
             name,
             show_intensity: false,
             show_cropping: true,
-            unknown_bl0: 0,
-            unknown_bl1: 0,
+            hidden_scans: Vec::new(),
+            hidden_regions: Vec::new(),
             stylization_type: 1,
             intensity_color_scheme: String::new(),
             current_color_scheme: String::new(),