trimmed_range_of_child broken for ≥3-level nesting

An LLM-based code review claims that there is a bug here:
> composition.cpp:397-400: missing current = parent; at the end of the loop (compare range_of_child at line 349). Stale current makes index_of_child fail with NOT_A_CHILD_OF for any timeline→track→stack→clip hierarchy. Poisons trimmed_range_in_parent, children_in_range, algo::fill.

Could we get a volunteer to verify if this is a real bug? It sounds plausible, and could be easily verified by writing a test case and/or a test OTIO file that creates the scenario described.

Start in composition.cpp around lines 349 and 397-400, comparing trimmed_range_of_child with range_of_child. Create a test case or test OTIO file covering a timeline→track→stack→clip hierarchy, then run the relevant tests. Done means confirming whether the reported NOT_A_CHILD_OF failure occurs and documenting the result.

Written by the indexing model from the issue text.