`JobQueue::cancel` decided whether a DAG could be cancelled by reading node
run-state, walked the subtree, judged per node whether that node had asked
to observe cancellation, and re-ran the container's roll-up. Every one of
those is a fact the scheduler owns; core was reaching across the boundary
to compute them.
`Scheduler::cancel_node` now takes the whole subtree: cancelling a node
cancels the work under it, since a group is abandoned by abandoning its
root. The existing method generalises rather than gaining a sibling — it
had one production caller, which this replaces.
The gate runs over the work *under* the node, not the node itself: a group
root's state is its subtree's roll-up rather than a step that ran, so a
container is `Finishing` and never `Pending`, and gating on it would refuse
every cancel. A node with no children is its own work, which keeps the
previous single-node behaviour.
`observes_cancellation` moves in with it — it reads a node's declared edges
and knows nothing about what the payload means.
Core keeps the one genuinely domain-specific step, resolving a wire
`dag_id` to its container node, and is three lines otherwise.
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