spec: coroutine primitive — make-coroutine/resume/yield via perform/cek-step-loop

spec/coroutines.sx: define-library with make-coroutine, coroutine-resume,
coroutine-yield, coroutine?, coroutine-alive?. Built on existing perform/
cek-step-loop/cek-resume suspension machinery.

spec/tests/test-coroutines.sx: 17 tests — multi-yield, final return,
arg passthrough, alive? predicate, nested coroutines, recursive iteration,
independent coroutine interleaving.

Key: coroutine body must use (define loop (fn…)) not named let — named let
transpiles to cek_call→cek_run which rejects IO suspension. All 17/17 pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

spec/primitives.sx

@@ -797,3 +797,5 @@
   :params ((source :as string))
   :returns "list"
   :doc "Parse SX source string into a list of AST expressions.")
+
+(define-module :stdlib.coroutines)