Function dispatch_if_async 

pub fn dispatch_if_async(
    callee: &Value,
    args: &[Value],
    env: &Env,
) -> Option<Value>

If callee is an ^:async Clojure function and an async runtime is registered, spawn its body as a task and return a Value::Future.

Returns None when there is no async runtime or the callee is not an async function, in which case the caller proceeds with the normal synchronous call path. This is the single dispatch point shared by apply_value here and eval_call in crate::interp.