The `:child_starter` seam took four values - parent run id, effect,
index, count - and the settlement side records the aggregation policy
on every child's own linkage, so a `first_error` fan-out could not be
expressed through it at all. RQ-031-4 resolved option (b): the seam
carries the option list, which is the same widening with room for the
next scheduling fact.

`c:StatifierOban.Invoke.ChildStarter.start_child/5` now takes
`opts`, carrying `policy: :all | :first_error`.
`StatifierOban.Invoke.FanOut` reads the policy off the `core.map`
invocation's own `on` parameter - `"all"`, `"first_error"`, or absent,
which is `:all` - and stamps it on every start job, which
`StatifierOban.Invoke.ChildStartWorker` reads back and hands the seam.
An `on` that is neither word refuses the fan-out on
`error.communication.invoke.<invoke_id>` rather than defaulting: the
two policies differ in whether a failing child cancels its siblings.

The policy is the chart's word, not the handler's, so it comes off the
effect rather than out of `{:fan_out, items, opts}`.

The callback's arity change is not breaking: `start_child/4` shipped
only in the untagged 0.7.0 prep, so no published release carries it,
and the CHANGELOG bullet lands in that section rather than in a
fragment the already-assembled prep would miss.

No cap, slice, or cancel behaviour changed.

Refs: sob-64p