Adds a `lineage-unlink` flag to triage parent deletes (FN-5131), enabling the split-into-subtasks path to break parent-child lineage on deletion. The primary addition is a comprehensive new test covering that split scenario, with minor adjustments to two existing lineage-related test files.
Fusion-Task-Id: FN-5131
Fusion-Task-Lineage: cb720e8f-8c49-42e4-873a-9949f3ba3427
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