Reorder children for panel display order (Feature 03, task 3)

- ChildrenDao.reorder now requires a COMPLETE permutation of the
  children table: partial lists (missing an existing child), unknown
  ids and duplicates are rejected with ValueError before any write,
  inside the existing transaction and connection lock (check and
  write atomic). Empty list is the maximally partial reorder against
  a non-empty table; against an empty table it stays a no-op. The two
  DAO tests that encoded the old permissive semantics (silent no-op
  on empty/unknown) are updated to the new contract.
- children.py gains reorder_children: the Feature 09-facing wrapper,
  which additionally rejects non-integer ids up front (bools and
  floats would silently compare against stored integer ids at the SQL
  layer).
- Business tests: round-trip (active list returns new order), partial/
  unknown/duplicate/non-int rejections with order-unchanged proofs,
  empty-list rejection, and inactive children keeping positions in a
  whole-table permutation. Suite: 866 passed.

tests/test_dao_children.py

@@ -227,24 +227,34 @@ def test_reorder_is_transactional_and_idempotent(tmp_path) -> None:
         _run(database.close())
 
 
-def test_reorder_empty_list_is_noop(tmp_path) -> None:
+def test_reorder_empty_list_is_rejected(tmp_path) -> None:
+    """An empty list is the maximally partial reorder: rejected while
+    any child exists (Feature 03 reorder contract)."""
+    import pytest as pytest_module
+
     database = _open_db(tmp_path / "reorder-empty.db")
     try:
         dao, _ = _daos(database)
         _make_child(dao, "Ada")
-        _run(dao.reorder([]))
+        with pytest_module.raises(ValueError, match="missing child ids"):
+            _run(dao.reorder([]))
         children = _run(dao.list_all())
         assert children[0].sort_order == 0
     finally:
         _run(database.close())
 
 
-def test_reorder_unknown_id_leaves_it_untouched(tmp_path) -> None:
+def test_reorder_unknown_id_is_rejected(tmp_path) -> None:
+    """An unknown id is rejected before any write; the order is
+    untouched (Feature 03 reorder contract)."""
+    import pytest as pytest_module
+
     database = _open_db(tmp_path / "reorder-unknown.db")
     try:
         dao, _ = _daos(database)
         a = _make_child(dao, "Ada")
-        _run(dao.reorder([a.id, 999]))
+        with pytest_module.raises(ValueError, match="unknown child ids"):
+            _run(dao.reorder([a.id, 999]))
         children = _run(dao.list_all())
         assert children[0].sort_order == 0
     finally: