* feat(core): add serialization support for workflow function references
Add a WorkflowFunction reducer/reviver pair so that workflow function
references (functions with a .workflowId property, as produced by the
SWC compiler) can cross the step serialization boundary.
Reducer: detects functions with a .workflowId string property, serializes
as { workflowId }. Only matches functions (not plain objects) to prevent
infinite recursion since the reduced form is a plain object.
Reviver: deserializes as a plain { workflowId } metadata object, which
start() already accepts via its WorkflowFunction | WorkflowMetadata
parameter type.
Also adds an observability reviver that renders as '<workflow:workflowId>'.
* fix: address review feedback - fix error assertion, add o11y reviver test
Nathan Rajlich