Function schema_for 

pub fn schema_for<T: JsonSchema>() -> Value

Generate a JSON Schema for T as a serde_json::Value.

The macro-generated #[workflow(params(...))] descriptor calls this per declared param type; accumulator/reactor boundary derivation (Task D) calls it per boundary type. Returns Value::Null only if the generated schema fails to serialize (not expected for a well-formed JsonSchema impl).