Review catch from argus on #2770: with the terminal hook gone, cancel
spares the DAG's tail node so it can report the cancellation — which
leaves that node `Pending` until the scheduler's next pass.
`post_rebuild_queue_cancel` emits its snapshot synchronously, and
`rollup_state` ranked `Queued` above `Cancelled`, so the operator who
just cancelled a DAG saw it go back to **Queued**: "the cancel didn't
take". Asserted in `cancel_clears_queued_dag`, which failed before this.
Fixing it surfaced an older disagreement. `builds.js::rollupState` has
always been `failed > cancelled > running > queued`; the Rust was
`failed > running > queued > cancelled`. The two had silently diverged
under a doc-comment claiming they agree. Harmless until now only because
cancelled nodes never coexisted with live ones — a spared tail running
over cancelled work would have rendered `Running` host-side and
`Cancelled` in the dashboard.
The JS was the correct side, so the Rust moves to match it exactly:
`Failed > Cancelled > Running > Queued > Done`. No frontend change.
36918e0432