Multi-shot delimited continuations: 868/870 passing

Continuations are now multi-shot — k can be invoked multiple times.
Each invocation runs the captured frames via nested cek-run and
returns the result to the caller's continuation.

Fix: continue-with-call runs ONLY the captured delimited frames
(not rest-kont), so the continuation terminates and returns rather
than escaping to the outer program.

Fixed 4 continuation tests:
- shift with multiple invokes: (list (k 10) (k 20)) → (11 21)
- k returned from reset: continuation callable after escaping
- invoke k multiple times: same k reusable
- k in data structure: store in list, retrieve, invoke

Remaining 2 failures: scope/provide across shift boundaries.
These need scope state tracked in frames (not imperative push/pop).

JS 747/747, Full 868/870, Python 679/679.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/static/scripts/sx-browser.js

@@ -14,7 +14,7 @@
   // =========================================================================
 
   var NIL = Object.freeze({ _nil: true, toString: function() { return "nil"; } });
-  var SX_VERSION = "2026-03-15T14:09:57Z";
+  var SX_VERSION = "2026-03-15T14:20:13Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }
@@ -1835,8 +1835,10 @@ PRIMITIVES["step-continue"] = stepContinue;
   var contData = continuationData(f);
   return (function() {
   var captured = get(contData, "captured");
-  var restK = get(contData, "rest-kont");
-  return makeCekValue(arg, env, concat(captured, restK));
+  return (function() {
+  var result = cekRun(makeCekValue(arg, env, captured));
+  return makeCekValue(result, env, kont);
+})();
 })();
 })() : (isSxTruthy((isSxTruthy(isCallable(f)) && isSxTruthy(!isSxTruthy(isLambda(f))) && isSxTruthy(!isSxTruthy(isComponent(f))) && !isSxTruthy(isIsland(f)))) ? makeCekValue(apply(f, args), env, kont) : (isSxTruthy(isLambda(f)) ? (function() {
   var params = lambdaParams(f);