docs(#2815): the transient pill's vocabulary is open, not a fixed set

argus flagged on !2910 that the dashboard doc still described the pill's
old shape. It did, in two ways that now teach the wrong thing:

- it listed a fixed vocabulary, where the label is now the running
  node's own wire tag — the `NodeKind::as_str` strings `NodeView.kind`
  already carries. A client that switches on specific values is now
  wrong, and `restarting` in particular no longer exists at all.
- it described the transient as operator-initiated ("set the moment the
  operator clicks"), which was the distinction between it and the
  rebuild-queue fallback. That is no longer true: the transient is
  derived from the running node, so worker-driven work the operator
  never clicked lights the same pill.

The queue-`kind` half of that section is untouched — that path did not
change and its vocabulary is still fixed.

Also notes on the wire-event list that `transient_kind` is a display
string to render, not an enum to branch on, since that is the property a
client would otherwise have to infer from a now-open set.

Docs only, no code change.

docs/web-ui/dashboard.md

@@ -852,12 +852,29 @@ fetch entirely.
   icon, so it's obvious at a glance which container is actually
   moving.
   **Pending-state derivation:** the pill is sourced from two
-  separate stores in priority order. (1) The operator-initiated
+  separate stores in priority order. (1) The **transient**
-  **transient** (`transientsState`) is set on the dashboard the
+  (`transientsState`) — covers the create-and-start window where the
-  moment the operator clicks start / stop / restart / rebuild /
-  destroy / spawn — covers the create-and-start window where the
   container literally isn't up yet, before any backend state event
-  has fired. (2) If no transient is set, the **rebuild-queue
+  has fired.
+
+  A transient is **derived from the job-queue node currently
+  running** against that agent, not declared per request, so its
+  label follows the operation as it progresses (a rebuild reads
+  `stop_for_update`, then `swap`, then `reconcile` rather than one
+  constant `rebuilding` for its whole life). Two consequences for
+  anything rendering it:
+
+  - The label vocabulary is **open** — it is the node's own wire tag
+    (`NodeKind::as_str`, the same strings `NodeView.kind` carries),
+    not a fixed set. Treat it as an opaque display string; do not
+    switch on specific values. `restarting` in particular no longer
+    exists, because no node kind is unique to a restart.
+  - It is **not** exclusively operator-initiated. Work the operator
+    never clicked (a meta-update cascade, a crash-recover rebuild)
+    lights the same pill, since it is the running node that sets it.
+
+  Ops with no queue node behind them (destroy, migration) supply
+  their own label directly. (2) If no transient is set, the **rebuild-queue
   entry** for this agent is consulted (`rebuildQueueState`); this
   covers worker-driven ops — meta-update cascades, crash-recover
   rebuilds, approval-driven rebuilds — that the operator didn't
@@ -1351,7 +1368,9 @@ payload):
 - `transient_set` (name, transient_kind, since_unix) /
   `transient_cleared` (name) — lifecycle action spinners. The
   client ticks the elapsed-seconds badge off `since_unix`
-  client-side, no polling.
+  client-side, no polling. `transient_kind` is an **open**
+  display string (the running node's own tag), not a fixed
+  enum — render it, don't branch on it.
 - `container_state_changed` (container: ContainerView) /
   `container_removed` (name) — per-row container mutations,
   emitted by `Coordinator::rescan_containers_and_emit` from