Isolation mode: double-clicking a single object isolates it

With Double Click To Isolate on, the Selection tool isolated groups only. A
double-click on a single path, compound path, image, mesh or placed file now
isolates it too, as the public documentation describes. Type still switches to
the Type tool; blends, envelopes, repeats and symbol instances keep their own
editing modes.

While a single object is isolated only it is hit, marqueed and taken by Select
All, new art goes beside it in its group or layer, and a double-click on the
empty canvas leaves with it selected. object.isolate accepts any object except
type.

crates/engine/src/cmd/select.rs

@@ -193,7 +193,8 @@ pub fn specs() -> Vec<CommandSpec> {
 fn selectable(d: &Document, iso: Option<NodeId>) -> Vec<NodeId> {
     match iso.and_then(|i| d.node(i)) {
         Some(c) if d.ancestry(c.id).is_some_and(|a| a.iter().all(|i| d.node(*i).is_some_and(|n| n.visible && !n.locked && !n.is_template()))) => {
-            c.layer_art(true)
+            // A single object isolated is all there is to select.
+            if c.is_container() { c.layer_art(true) } else { vec![c.id] }
         }
         Some(_) => vec![],
         None => d.selectable_art(),