feat(v0): show reusable partial feedback

Add a generated todo notice slot to the v0 normal path so initial render, append, replace, remove, and dynamic Vec<IntoEffect> refreshes visibly compose one reusable TodoRow partial with adjacent generated UI feedback.

req: canonical_authoring/003

examples/v0/templates/todos.heml

@@ -4,6 +4,7 @@
     <button type="submit">Add</button>
   </form>
   <p data-hemx-slot="summary">{+ self.summary +}</p>
+  <p data-hemx-slot="notice">{+ self.notice +}</p>
   <ul data-hemx-slot="todo_row">
     <template h-for="row in &self.rows" h-key="row.id">
       {+ row +}