scheme: Phase 5a — call/cc + 8 tests

scheme-standard-env binds:
- call/cc                            — primary
- call-with-current-continuation     — alias

Implementation wraps SX's host call/cc, presenting the captured
continuation k as a Scheme procedure that accepts a single value
(or a list of values for multi-arg invocation). Single-shot
escape semantics: when k is invoked, control jumps out of the
surrounding call/cc form. Multi-shot re-entry isn't safely
testable without delimited-continuation infrastructure (the
captured continuation re-enters indefinitely if invoked after
the call/cc returns) — deferred to a follow-up commit if needed.

Tests cover:
- No-escape return value
- Escape past arithmetic frames
- Detect/early-exit idiom over for-each
- Procedure? on the captured k

220 total Scheme tests now (62 + 23 + 49 + 78 + 8).

lib/scheme/runtime.sx

@@ -510,4 +510,25 @@
         "equal?"
         (scm-binary "equal?" (fn (a b) (= a b))))
       (scheme-env-bind! env "eq?" (scm-binary "eq?" (fn (a b) (= a b))))
+      ;; ── call/cc (R7RS first-class continuations) ────────────
+      ;; Captures the host SX continuation, wraps it as a Scheme
+      ;; procedure (fn (vargs) ...) and passes it to the user proc.
+      ;; Calling the captured k with one value re-enters the
+      ;; continuation; with multiple values, passes them as a list.
+      (scheme-env-bind! env "call/cc"
+        (fn (args)
+          (cond
+            ((not (= (length args) 1))
+              (error "call/cc: expects 1 argument"))
+            (:else
+              (call/cc
+                (fn (k)
+                  (let ((scheme-k
+                          (fn (vargs)
+                            (cond
+                              ((= (length vargs) 1) (k (first vargs)))
+                              (:else (k vargs))))))
+                    (scheme-apply (first args) (list scheme-k)))))))))
+      (scheme-env-bind! env "call-with-current-continuation"
+        (refl-env-lookup env "call/cc"))
       env)))

lib/scheme/tests/control.sx

@@ -0,0 +1,72 @@
+;; lib/scheme/tests/control.sx — call/cc, dynamic-wind, exceptions.
+
+(define scm-ctl-pass 0)
+(define scm-ctl-fail 0)
+(define scm-ctl-fails (list))
+
+(define
+  scm-ctl-test
+  (fn
+    (name actual expected)
+    (if
+      (= actual expected)
+      (set! scm-ctl-pass (+ scm-ctl-pass 1))
+      (begin
+        (set! scm-ctl-fail (+ scm-ctl-fail 1))
+        (append! scm-ctl-fails {:name name :actual actual :expected expected})))))
+
+(define
+  scm-ctl
+  (fn (src) (scheme-eval (scheme-parse src) (scheme-standard-env))))
+
+(define
+  scm-ctl-all
+  (fn
+    (src)
+    (scheme-eval-program (scheme-parse-all src) (scheme-standard-env))))
+
+;; ── call/cc — escape continuations ──────────────────────────────
+;; Single-shot only: when k is invoked, control jumps out of the
+;; surrounding call/cc and the result of the entire call/cc form is
+;; whatever was passed to k.
+
+(scm-ctl-test
+  "call/cc: no escape"
+  (scm-ctl "(call/cc (lambda (k) 42))")
+  42)
+(scm-ctl-test
+  "call/cc: simple escape"
+  (scm-ctl "(call/cc (lambda (k) (+ 1 (k 42))))")
+  42)
+(scm-ctl-test
+  "call/cc: escape past *"
+  (scm-ctl "(+ 10 (call/cc (lambda (k) (* 2 (k 5)))))")
+  15)
+(scm-ctl-test
+  "call/cc: alias call-with-current-continuation"
+  (scm-ctl "(call-with-current-continuation (lambda (k) (k 99)))")
+  99)
+(scm-ctl-test
+  "call/cc: doesn't escape if k unused"
+  (scm-ctl "(+ 1 (call/cc (lambda (k) (* 100 1))))")
+  101)
+
+;; ── call/cc as early-exit for list search ───────────────────────
+(scm-ctl-test
+  "call/cc: detect-via-escape"
+  (scm-ctl-all
+    "(define (detect pred xs)\n       (call/cc\n         (lambda (return)\n           (for-each\n             (lambda (x) (if (pred x) (return x) #f))\n             xs)\n           #f)))\n     (detect (lambda (x) (> x 10)) '(1 5 7 12 20))")
+  12)
+(scm-ctl-test
+  "call/cc: detect returns #f when no match"
+  (scm-ctl-all
+    "(define (detect pred xs)\n       (call/cc\n         (lambda (return)\n           (for-each\n             (lambda (x) (if (pred x) (return x) #f))\n             xs)\n           #f)))\n     (detect (lambda (x) (> x 100)) '(1 5 7))")
+  false)
+
+;; ── call/cc producing the captured k value ──────────────────────
+(scm-ctl-test
+  "call/cc: k is a procedure"
+  (scm-ctl "(procedure? (call/cc (lambda (k) k)))")
+  true)
+
+(define scm-ctl-tests-run! (fn () {:total (+ scm-ctl-pass scm-ctl-fail) :passed scm-ctl-pass :failed scm-ctl-fail :fails scm-ctl-fails}))