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/lifecycle/mod.rs

@@ -273,11 +273,9 @@ pub async fn swap_update(
     name: &str,
     hive: &HiveEnv,
     paths: &AgentPaths,
-    on_step: &(dyn Fn(&str) + Send + Sync),
     on_build_log_id: &(dyn Fn(i64) + Send + Sync),
 ) -> Result<()> {
     write_dropins(name, hive, paths).await?;
-    on_step("nixos-container update");
     priv_run_inner("update", name, Some(on_build_log_id)).await
 }