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)))