Fix double node creation when picking a palette entry by click

- TextField: new onSubmit callback (Enter only, independent of value change)
- NodePalette: use onSubmit instead of onChange, add per-open picked guard
- e2e: new check 13a (type + click row -> exactly one node), wait for
  Chromium exit in finally with SIGKILL escalation after 5s

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/editor/graph/NodePalette.ts

@@ -8,6 +8,8 @@ export class NodePalette extends Popup {
   private readonly search: TextField;
   private readonly list = new ListView();
   private filtered: NodeDef[] = [];
+  /** Guards against a second pick per opening (e.g. Enter and a row click racing). */
+  private picked = false;
 
   constructor(private readonly onPick: (def: NodeDef) => void) {
     super({ padding: 4, gap: 4 });
@@ -15,7 +17,9 @@ export class NodePalette extends Popup {
     this.search = new TextField('', {
       placeholder: 'Search nodes…',
       onInput: () => this.refresh(),
-      onChange: () => {
+      // Enter only. NOT onChange: that also fires when the search field loses focus because
+      // the user clicked a list row, which would add the first match *and* the clicked one.
+      onSubmit: () => {
         const first = this.filtered[0];
         if (first) this.pick(first);
       },
@@ -36,6 +40,7 @@ export class NodePalette extends Popup {
 
   override open(globalX: number, globalY: number): void {
     super.open(globalX, globalY);
+    this.picked = false;
     ui().focus.focus(this.search);
   }
 
@@ -49,6 +54,8 @@ export class NodePalette extends Popup {
   }
 
   private pick(def: NodeDef): void {
+    if (this.picked) return;
+    this.picked = true;
     this.close();
     this.onPick(def);
   }