feat(examples): expose v0 todo field error targets

Add generated field-error targets to the v0 add and rename forms so the canonical todo example shows recoverable typed form feedback end to end instead of emitting errors with no visible target.

req: examples/001

req: canonical_authoring/003

examples/v0/templates/todos.heml

@@ -2,6 +2,7 @@
   <form data-hemx-handle="add_todo" data-hemx-form="new_todo">
     <input name="title" required="required">
     <button type="submit">Add</button>
+    <p data-hemx-error-for="title"></p>
   </form>
   <p data-hemx-slot="summary">{+ self.summary +}</p>
   <p data-hemx-slot="notice">{+ self.notice +}</p>