fix(#2756): a cancelled DAG must not read back as Queued

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.

hive-c0re/src/job_queue/tests.rs

@@ -934,6 +934,11 @@ fn cancel_clears_queued_dag() {
     let q = JobQueue::new(1);
     let id = submit(&q, rebuild("agent-a", "r"));
     assert!(q.cancel(id), "fully-queued dag cancels");
+    // The operator sees `Cancelled` the moment the cancel returns — the spared
+    // tail is still `Pending`, and a DAG must not read `Queued` back to the
+    // operator who just cancelled it (the dashboard renders this roll-up from
+    // the snapshot `post_rebuild_queue_cancel` emits synchronously).
+    assert_eq!(state_of(&q, id), State::Cancelled, "no stale Queued gap");
     // Every work node is `Cancelled`, but the tail is spared so it can still
     // report the cancellation — so it is the one thing left to claim.
     let tail = claim_one(&q);