Aligns each anchored portion of the input dataset onto a common anchor timestamp while preserving grouping
tags. Without a reference the selected anchor becomes timestamp zero, so the output is in a relative time
domain; with a reference the anchors are aligned onto the referenced portion’s anchor and the output stays
in the input’s absolute time domain. Currently run boundary and event anchors are supported, and every
storage-backed leaf of the input must resolve to run-backed branches.