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

admin/src/components/form/formContext.ts

@@ -82,5 +82,12 @@ export const FORM_SESSION: InjectionKey<FormSession> = Symbol('summer.form.sessi
 /** Event a widget element dispatches to run its action (bubbles, composed). */
 export const WIDGET_EVENT = 'summer-action'
 
+/**
+ * Event the SPA dispatches on the plugin element after each successful action,
+ * with detail {data, fill, message}: the response data as returned (undefined
+ * when the answer carried none), the fill object and the message.
+ */
+export const WIDGET_RESULT_EVENT = 'summer-result'
+
 /** How long a widget waits for its custom element to be defined. */
 export const WIDGET_TIMEOUT = 5000