The operations the cutover removed come back in their Petri shape, with
no engine of their own: the timeline is the run's checkpoint records
labelled by stage (`RunTimeline::build`), and a target (`@ordinal`, a
node, or `node@visit`) resolves to one of them. A fork's run row is the
source's spec under a new id with `fork_source_ref` naming the source and
the checkpoint's commit (`persist_forked_run`), and `retried_from` when
it is a retry. A rewind needs a terminal source and records
`run.superseded` on it; a retry needs a terminal source and reruns the
last checkpointed stage when the run failed on it.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>