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/lib.rs

@@ -443,9 +443,10 @@ pub struct Session {
     untitled_counter: u32,
     /// Session-level state of the menu commands (saved selections, guide lock).
     pub(crate) menu: cmd::menucmds::MenuState,
-    /// The selected gradient stop (`gradient.selectStop`), shared by the Gradient tool's annotator,
-    /// the Gradient and Color panels and agents.
-    pub gradient_stop: Option<usize>,
+    /// The selected gradient stop (`gradient.selectStop`) and whose gradient it was selected on,
+    /// shared by the Gradient tool's annotator, the Gradient and Color panels and agents: read it
+    /// with [`Session::selected_stop`].
+    pub(crate) gradient_stop: Option<(usize, cmd::gradient::StopOwner)>,
 }
 
 impl Default for Session {