fix(runtime): show transport failures in app error outlets

Make failed hemx requests visibly recoverable by updating root-scoped data-hemx-error outlets while preserving fail-closed non-2xx handling and inspectable hemx:error events. The v0 generated-app exemplar now includes a transport error outlet alongside generated field-error targets.

req: runtime/005

req: examples/001

req: canonical_authoring/003

examples/v0/templates/todos.heml

@@ -4,6 +4,7 @@
     <button type="submit">Add</button>
     <p data-hemx-error-for="title"></p>
   </form>
+  <p data-hemx-error role="status" hidden="hidden"></p>
   <p data-hemx-slot="summary">{+ self.summary +}</p>
   <p data-hemx-slot="notice">{+ self.notice +}</p>
   <ul data-hemx-slot="todo_row">