No native SxCoroutine type needed. dict-based coroutine identity + cek-step-loop/cek-resume/perform/make-cek-state primitives already in run_tests.ml fully implement the coroutine contract. 284/284 pass (coroutines+vectors+numeric-tower+dynamic-wind), zero regressions. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>