refactor(selection): split 31-field SelectionState into five sub-structs

SelectionState now holds view, gesture, input, menu and orbit sub-structs; all 450 access sites updated mechanically with no behavior change. Deletes three write-only dead fields (box_last, box_last_crossing, poly_last_crossing). Hand-rolled clears in commands and dim_viewport route through the new clear methods; bench fixture updated. Full suite: 1976 passed / 0 failed / 25 ignored.

src/app/command_driver/mod.rs

@@ -1148,7 +1148,7 @@ impl OpenCADStudio {
         // The command picked the path; the hit test lives here, where
         // the camera and the drawing's geometry are. Everything after
         // matches what a lasso does, Remove included.
-        let canvas = self.tabs[i].scene.selection.borrow().vp_size;
+        let canvas = self.tabs[i].scene.selection.borrow().view.vp_size;
         let bounds = iced::Rectangle {
             x: 0.0,
             y: 0.0,

src/app/command_driver/utilities.rs

@@ -364,7 +364,7 @@ impl OpenCADStudio {
             .is_some_and(|command| command.wants_point_pick_context())
         {
             let scene = &self.tabs[i].scene;
-            let size = scene.selection.borrow().vp_size;
+            let size = scene.selection.borrow().view.vp_size;
             let edit = scene.viewport_edit_frame(size);
             let tile = edit
                 .as_ref()
@@ -919,8 +919,8 @@ impl OpenCADStudio {
             let crossing = matches!(kw.as_str(), "C" | "CROSSING");
             {
                 let mut selection = self.tabs[i].scene.selection.borrow_mut();
-                selection.box_crossing = crossing;
-                selection.box_crossing_locked = true;
+                selection.gesture.box_crossing = crossing;
+                selection.gesture.box_crossing_locked = true;
             }
             let hint = if crossing {
                 crate::t!("Crossing: specify first corner.")