One-at-a-time claiming is what lets a caller choose between claiming again
immediately and backing off — a batch return cannot express that choice,
and the choice is the point: the run loop wants to know there was work
before it decides whether to wait.
`settle()` keeps its exact meaning as `while let Some(id) = claim_one()`.
A node started by an earlier iteration is `Running`, not terminal, so it
cannot satisfy another node's dependency in the same sweep; it only
consumes resources. The crate's ~45 existing `settle()` assertions — which
cover resource borrowing, cap-1 serialisation, roll-up and cancellation —
are what verify that equivalence, so it is checked rather than argued.
`None` means "nothing runnable right now", which is deliberately a
different statement from "nothing pending": a node can be pending and
unrunnable because its resources are held elsewhere.
Cost stated rather than left to be found: each `claim_one` rescans the
pending set, so `settle` is O(n^2) in nodes claimed where the single-pass
version was O(n). The graph is bounded by history retention.
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