feat(hemx): reuse todo row partials

Make the canonical v0 todo flow use one TodoRow partial shape for initial list rendering and generated keyed append/replace/remove helpers. Teach hemx-build to treat collection hosts with keyed h-for children as keyed targets, so beginner templates can keep the DRY outer slot + child partial composition.

req: canonical_authoring/002

req: canonical_authoring/003

req: examples/004

req: list/001

examples/v0/templates/todos.heml

@@ -4,20 +4,9 @@
     <button type="submit">Add</button>
   </form>
   <p data-hemx-slot="summary">{+ self.summary +}</p>
-  <div data-hemx-slot="todo_list">
-    <ul data-hemx-slot="todo_row">
-      <template h-for="todo in &self.items" h-key="todo.id">
-        <li data-hemx-slot="todo_row" +data-key="todo.id">
-          <span>{+ todo.title +}</span>
-          <form data-hemx-handle="rename_todo">
-            <input type="hidden" name="id" +value="todo.id">
-            <button type="submit" name="title" value="Renamed todo">Rename</button>
-          </form>
-          <form data-hemx-handle="delete_todo">
-            <button type="submit" name="id" +value="todo.id">Delete</button>
-          </form>
-        </li>
-      </template>
-    </ul>
-  </div>
+  <ul data-hemx-slot="todo_row">
+    <template h-for="row in &self.rows" h-key="row.id">
+      {+ row +}
+    </template>
+  </ul>
 </section>