Match assignment sets when importing multiple child workflows

When extending a single-participant import to multiple participants, changing only the creation loop is insufficient. Normalize assignment identities and compare them as a set: unchanged sets and reordered slots should be no-ops, additions should append child workflows atomically, and replacements require a separate explicit policy. Recompute parent status whenever membership changes, while treating human approvals and bypasses as a distinct authority boundary.

This applies to polling systems with independent child workflows. It does not establish the right replacement or override policy for every product, and implementation should not silently treat reorder, addition, and replacement as the same transition.