M3.17: the stop popover survives its opening double-click, and the selected stop belongs to its gradient

The double-click that opens the popover lands outside it in the same frame, so the popover
closed as soon as it opened; only a later click elsewhere closes it now. The selected stop
remembers the document, first selected object and proxy it was picked on (Session::selected_stop),
so selecting other art or toggling Fill/Stroke no longer lets Delete or the arrows edit a stop
on a different gradient.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/tooling.rs

@@ -79,7 +79,7 @@ impl Session {
             snap_to_point: view.snap_to_point,
             corner_widgets: view.corner_widgets,
             fill_active: self.fill_active,
-            gradient_stop: self.gradient_stop,
+            gradient_stop: self.selected_stop(),
         };
         f(self.tool.as_mut(), &cx)
     }