refactor(#2449): dashboard + docs for the SetWanted node

- builds.js NODE_KIND_LABEL: 'set wanted' (and the previously-missing
  'noop') so the new head node renders with a label, not the raw kind.
- docs/coordinator.md + templates.rs module doc: the power-op DAG shapes
  now show the head SetWanted node; SetWanted added to the lease-needing
  list with the atomicity rationale.

frontend/packages/dashboard/src/builds.js

@@ -154,6 +154,8 @@ const NODE_KIND_LABEL = {
   write_dropin: 'dropin',
   write_perm_file: 'perm file',
   approval_deploy: 'deploy',
+  set_wanted: 'set wanted',
+  noop: 'noop',
 };
 
 // The currently-running node of a DAG (per-node `step` labels + build