job_queue: retire the now-off-wire step sub-step label

The `step` label was taken off the wire in #2661, when each deploy phase
became a first-class DAG node. Since then it has been written but never
read: `NodeRuntime` derives only `Debug, Default, Clone` — no serde — so
the field could not reach any client, and the only reads of it were the
dedup checks inside its own setters. This deletes the machinery.

Removed:

- `NodeRuntime.step`, `set_step`, `set_step_running`, and the
  `rt.step = None` clear in `complete_node`. `NodeRuntime` keeps its
  remaining `build_log_id` field (deliberately still a struct — collapsing
  it to a bare `Option<i64>` would churn every call site for no gain).
- `Ctx::step` and its ~15 call sites in `job_queue/exec.rs`. `Ctx` itself
  stays: it is the build-log sink, which `run_prebuild` and `run_swap`
  still use.
- `Coordinator::set_queue_step` and its 11 callers in `actions.rs`.
- `JobQueue::running_node_of`, reachable only from `set_queue_step`.
- `swap_update`'s `on_step` parameter and its one body call.
- The `set_step_only_on_running_and_signals_change` test.

Dropping the calls orphaned parameters, which are removed with their call
sites: `ctx` on ten executors that used it only as a step sink, and
`queue_entry_id` on `run_deploy_merge_verify` / `run_deploy_apply` /
`run_finalize_deploy` plus both `coord` and `queue_entry_id` on
`prepare_applied_target`. `run_deploy_tail` KEEPS its `queue_entry_id` —
that one has a genuine surviving use (the build-log link in the failure
comment posted to the PR).

One behavioural change, called out so it is not mistaken for a dropped
dashboard refresh: `Ctx::step` and `set_queue_step` each emitted a
`rebuild_queue_changed` snapshot when the label changed, and those
emissions go away with them. This is safe — the snapshot payload has no
step field, so those pushes carried nothing a client could observe. Real
state transitions still emit from the scheduler's claim and completion
paths, from `submit`, and from the three `actions.rs` sites. Net effect is
strictly fewer redundant SSE pushes.

Docs: `docs/coordinator.md` still listed `step` as a `NodeView` wire field
and `docs/web-ui/dashboard.md` documented a cyan `↳ <step>` sub-line under
each queue row. Neither has existed since #2661 — both corrected here, plus
the `job_queue/model.rs` module doc.

Not touched: `frontend/packages/dashboard/src/system-sections.css` has a
dead `.rqe-step` rule with no JS referencing it. Left for the frontend
owner rather than deleted here.

Closes: #2664

hive-c0re/src/job_queue/tests.rs

@@ -1134,29 +1134,7 @@ fn deploy_dag_skips_apply_but_still_runs_tail_when_verify_fails() {
     assert_eq!(summary.approval_id, Some(9));
 }
 
-// ---- steps, build logs, history ----
-
-#[test]
-fn set_step_only_on_running_and_signals_change() {
-    let q = JobQueue::new(1);
-    let id = submit(&q, rebuild("agent-a", "r"));
-    assert!(
-        !q.set_step_running(id, "too early"),
-        "no running node yet → refused"
-    );
-    let c = claim_one(&q);
-    assert!(q.set_step(id, c.node_id, "nix build"));
-    assert!(
-        !q.set_step(id, c.node_id, "nix build"),
-        "same label → false"
-    );
-    assert!(q.set_step(id, c.node_id, "next phase"));
-    assert!(q.set_step_running(id, "via running lookup"));
-    // `step` is host-side only now (off the wire); completion clears it
-    // internally, but there's no wire field to observe — the return-value
-    // contract above (running-gating + change signalling) is the behaviour.
-    q.complete_node(id, c.node_id, Ok(()));
-}
+// ---- build logs, history ----
 
 #[test]
 fn set_build_log_id_links_running_node() {