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/control/vision.rs

@@ -604,7 +604,7 @@ mod tests {
         app.automation_op(r#"{"op":"run","cmd":"LINE -10,-10 10,10"}"#);
 
         // Set test canvas viewport size
-        app.tabs[0].scene.selection.borrow_mut().vp_size = (800.0, 600.0);
+        app.tabs[0].scene.selection.borrow_mut().view.vp_size = (800.0, 600.0);
         app.tabs[0].scene.fit_all();
 
         let grounding = compute_grounding(&app, 800.0, 600.0, 800, 600);