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/partials/todo_items.heml

@@ -1,3 +0,0 @@
-<template h-if="!self.items.is_empty()">
-  <li h-for="todo in &self.items" +data-key="todo.id">{+ todo.title +}</li>
-</template>

examples/v0/templates/partials/todo_row.heml

@@ -1,10 +1,10 @@
-<li>
+<li +data-key="self.id">
   <span>{+ self.title +}</span>
-  <form data-hemx-handle="rename_todo_row">
+  <form data-hemx-handle="rename_todo">
     <input type="hidden" name="id" +value="self.id">
     <button type="submit" name="title" value="Renamed todo">Rename</button>
   </form>
-  <form data-hemx-handle="delete_todo_row">
+  <form data-hemx-handle="delete_todo">
     <button type="submit" name="id" +value="self.id">Delete</button>
   </form>
 </li>