feat(workflows): honor max_concurrency in fan-out via a bounded thread pool

src/specify_cli/workflows/base.py

@@ -97,6 +97,13 @@ class StepBase(ABC):
 
     Every step type — built-in or extension-provided — implements this
     interface and registers in ``STEP_REGISTRY``.
+
+    Thread-safety: ``STEP_REGISTRY`` holds a single shared instance per type, so
+    a concurrent ``fan-out`` (``max_concurrency > 1``) can invoke ``execute`` on
+    the same instance from several threads at once. Implementations must be
+    stateless / thread-safe — derive all per-run state from the ``config`` and
+    ``context`` arguments and never mutate ``self`` in ``execute``. The built-in
+    steps follow this rule.
     """
 
     #: Matches the ``type:`` value in workflow YAML.