feat(admin): widget field payload and summer-result data channel (quick-261006-eyj)

- WidgetField posts the summer-action event's detail.payload as payload only
  when the detail carries one; a payload-less post body is unchanged
- after a successful action the response data is set on the element as the
  data attribute (removed when the answer has none) and announced with a
  summer-result event carrying {data, fill, message}; failures dispatch nothing
- WIDGET_RESULT_EVENT exported from formContext; unit tests for both directions
- modules/boardwalk/dist rebuilt

modules/boardwalk/dist/index.html

@@ -6,7 +6,7 @@
     <meta name="robots" content="noindex, nofollow" />
     <meta name="summer-admin-base" content="__SUMMER_ADMIN_BASE__" />
     <title>SummerCMS</title>
-    <script type="module" crossorigin src="./assets/index-z_LzMU8D.js"></script>
+    <script type="module" crossorigin src="./assets/index-CzWk4rvA.js"></script>
     <link rel="stylesheet" crossorigin href="./assets/index-CqzF_Nki.css">
   </head>
   <body>