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/ui/popup/context_menu.rs

@@ -34,7 +34,7 @@ pub const MENU_WIDTH_WITH_HINTS: f32 = 236.0;
 pub const RECENT_LIMIT: usize = 10;
 
 /// Identifies an inline-expanding (accordion) submenu. Only one is open at a
-/// time; the open one is tracked in `SelectionState::context_menu_ui`.
+/// time; the open one is tracked in `SelectionMenu::ui`.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum SubmenuId {
     RecentInput,