`submit` used to complete the container node by hand, right after
inserting it, so it would park in `Finishing` and its children unblock.
That was the last caller of `Scheduler::complete` outside the crate, and
the justification was that the container "never needs claiming or
executing".
It does, though, in the sense that matters: it is a node with no logic of
its own, and the scheduler already knows what to do with one. It declares
no resources, so it is claimable the moment it is inserted; `run_node`'s
`Dag` arm already returns `Ok(())`, exactly as it does for `DeployWindow`,
which is the same shape and was never special-cased. Deleting the inline
completion costs one claim round-trip and removes the only reason the
crate had to expose completion at all.
`complete` is `pub(crate)` now. Completion is reachable only from inside
the future `claim_next` hands back, so a node cannot be finished without
the claim it answers, and cannot be claimed without the future that
finishes it. That was the point of the seam.
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