Merge origin/main into plugin/host-model-api

Conflicts were additive: main's AddLayer/ModifyLayer (LayerConfig, OptHandle)
alongside this branch's v7 system-variable, transaction, selection and solid
operations. Main's variants keep their positions in the IPC enums; the v7
additions follow them. src/io/mod.rs takes main's reworded Shape-rotation note.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/update/mod.rs

@@ -401,6 +401,7 @@ impl OpenCADStudio {
         {
             self.snapper.clear_tracking();
             self.otrack_active = None;
+            self.otrack_cross = None;
             self.otrack_kind = None;
         }
         if let Some((started, tab_id, before)) = perf_edit_before {
@@ -437,6 +438,7 @@ impl OpenCADStudio {
     pub(in crate::app) fn reset_tracking_after_point(&mut self) {
         self.snapper.clear_tracking();
         self.otrack_active = None;
+        self.otrack_cross = None;
         self.otrack_kind = None;
     }
 
@@ -4017,6 +4019,7 @@ impl OpenCADStudio {
                 if !self.snapper.otrack_enabled {
                     self.snapper.clear_tracking();
                     self.otrack_active = None;
+                    self.otrack_cross = None;
                     self.otrack_kind = None;
                 }
                 Task::none()