[iTerminate]

…. Closes #1688

[iTerminate]

I investigated the implications of removing the one-level representation constraint. Multilevel resolution appears structurally supported, but this change exposes several integrity risks that should be addressed or explicitly tested before merge:

• A represented parent can currently be unlinked while it still has represented children. Clearing the parent reference has no descendant validation, so those children can retain references into an assembly the parent no longer represents.
• Catalog element removal/replacement constraints do not consider representsItemElementList. Also, represents_item_element_id has no database foreign key, so deleting a represented catalog element may leave dangling references.
• /PromoteMachineElement accepts an independently supplied element and only checks that it belongs to some catalog/inventory item. It does not verify membership in the requested parent's assigned assembly or prevent duplicate sibling representation.
• No circular-reference validation was found. Multilevel representation may expose catalog assembly cycles through hierarchy traversal.
• Template propagation and catalog assembly swaps need explicit three-level tests.

At minimum, I recommend blocking unlink of a represented node with represented children, validating REST promotion against the parent's available elements, and adding tests for three-level hierarchies with/without templates, middle-node unlink, catalog element removal/replacement, assembly swap, and circular references.