plan(textarea-editor): fix stale jar reference in autocomplete

The Task 9 plan template used the captured \`jar\` closure variable in
acceptCompletion, which becomes stale after setLanguage's
tear-down-and-remount. Same class of bug Task 4's review caught and
fixed. Update the plan to match the correct implementation.

docs/superpowers/plans/2026-05-16-textarea-code-editor.md

@@ -1024,7 +1024,10 @@ function acceptCompletion(entry) {
   sel.removeAllRanges();
   sel.addRange(range);
   // Force CodeJar to re-highlight + emit onUpdate.
-  jar.updateCode(jar.toString());
+  // Use instance.jar (not bare `jar` closure) for consistency with
+  // setLanguage's tear-down-and-remount — the jar reference can be
+  // swapped at runtime.
+  instance.jar.updateCode(instance.jar.toString());
   hidePopup();
 }