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/cmd/mod.rs

@@ -15,7 +15,7 @@ mod edit;
 mod effectcmd;
 pub mod fileio;
 mod fonts;
-mod gradient;
+pub(crate) mod gradient;
 pub(crate) mod graph;
 pub mod help;
 mod layer;