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/scene/parametric_constraints.rs

@@ -3068,7 +3068,7 @@ impl super::Scene {
     /// `(id, anchor, outward_screen_direction, label, is_conflicting,
     /// hover_points)` for every enabled, visible constraint whose glyph
     /// projects on-screen.
-    /// `vp_size` is the full canvas size (as `SelectionState::vp_size`
+    /// `vp_size` is the full canvas size (as `SelectionView::vp_size`
     /// reports it), matching what `viewport_edit_frame`/
     /// `active_model_tile_bounds` expect. Mirrors the projection
     /// `crate::app::view` builds its own render list with. The memoised
@@ -3808,7 +3808,7 @@ mod tests {
                 Vector3::new(1.0, 0.0, 0.0),
             ),
         ));
-        scene.selection.borrow_mut().vp_size = (800.0, 600.0);
+        scene.selection.borrow_mut().view.vp_size = (800.0, 600.0);
         let id = scene.parametric_constraint_set_mut(scope).add(
             ConstraintKind::Horizontal,
             vec![ParametricRef::whole(line)],
@@ -3848,7 +3848,7 @@ mod tests {
                 Vector3::new(1.0, 0.0, 0.0),
             ),
         ));
-        scene.selection.borrow_mut().vp_size = (800.0, 600.0);
+        scene.selection.borrow_mut().view.vp_size = (800.0, 600.0);
         let id = scene.parametric_constraint_set_mut(scope).add(
             ConstraintKind::Horizontal,
             vec![ParametricRef::whole(line)],
@@ -3878,7 +3878,7 @@ mod tests {
                 Vector3::new(1.0, 0.0, 0.0),
             ),
         ));
-        scene.selection.borrow_mut().vp_size = (800.0, 600.0);
+        scene.selection.borrow_mut().view.vp_size = (800.0, 600.0);
         let id = scene.parametric_constraint_set_mut(scope).add(
             ConstraintKind::Horizontal,
             vec![ParametricRef::whole(line)],