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>