A workflow version names its engine with `engine = "petri"` in the
`[workflow]` table of `workflow.toml`, and `[server.execution] engine`
(`FABRO_SERVER_ENGINE`, `--engine`) defaults it for every version that
names none. The choice, with what Petri admitted (the lowered root graph
and its children by blob and digest), is recorded on the run spec as
`RunEngine`, carried on `run.created`, and replayed into the projection.
A legacy run's spec omits the field, so existing specs decode unchanged.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>