documents: an open block stays open while an overlay borrows focus

A menu or dialog took focus, the open block wrote back and closed, and
the overlay handed focus back to a closed input: its command reached
nothing and the selection was gone. The block now waits for the overlay
to let focus go, and writes back only when focus lands elsewhere.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/documents/document/mod.rs

@@ -80,6 +80,8 @@ struct Open {
     input: Entity<TextInput>,
     /// Opened past the end, to add a block.
     append: bool,
+    /// Watches an overlay that borrowed focus.
+    lent: Option<Subscription>,
     _subscriptions: Vec<Subscription>,
 }