Fix letrec thunk resolution + compiler letrec support + closure JIT check

Root cause: sf-letrec returns a thunk (for TCO) but the CEK dispatch
wrapped it as a value without evaluating. The thunk leaked as the
return value of letrec expressions, breaking sx-parse and any function
using letrec.

Fix: step-sf-letrec unwraps the thunk into a CEK state, so the last
letrec body expression is properly evaluated by the CEK machine.

Also:
- compile-letrec: two-phase (nil-init then assign) for mutual recursion
- Skip JIT for inner functions (closure.bindings != globals) in both
  vm_call and JIT hook
- vm-reset-fn for sx-parse removed (no longer needed)
- Parser regression test: letrec with mutable pos + recursive sublists

Test results: JS 943/17, OCaml 955/0, Python 747/0

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 = "2026-03-23T18:52:19Z";
+  var SX_VERSION = "2026-03-23T20:57:55Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }
@@ -1397,6 +1397,13 @@ PRIMITIVES["qq-expand"] = qqExpand;
 })(); };
 PRIMITIVES["sf-letrec"] = sfLetrec;
 
+  // step-sf-letrec
+  var stepSfLetrec = function(args, env, kont) { return (function() {
+  var thk = sfLetrec(args, env);
+  return makeCekState(thunkExpr(thk), thunkEnv(thk), kont);
+})(); };
+PRIMITIVES["step-sf-letrec"] = stepSfLetrec;
+
   // sf-dynamic-wind
   var sfDynamicWind = function(args, env) { return (function() {
   var before = trampoline(evalExpr(first(args), env));
@@ -1487,7 +1494,7 @@ PRIMITIVES["step-eval"] = stepEval;
   var args = rest(expr);
   return (isSxTruthy(!isSxTruthy(sxOr((typeOf(head) == "symbol"), (typeOf(head) == "lambda"), (typeOf(head) == "list")))) ? (isSxTruthy(isEmpty(expr)) ? makeCekValue([], env, kont) : makeCekState(first(expr), env, kontPush(makeMapFrame(NIL, rest(expr), [], env), kont))) : (isSxTruthy((typeOf(head) == "symbol")) ? (function() {
   var name = symbolName(head);
-  return (isSxTruthy((name == "if")) ? stepSfIf(args, env, kont) : (isSxTruthy((name == "when")) ? stepSfWhen(args, env, kont) : (isSxTruthy((name == "cond")) ? stepSfCond(args, env, kont) : (isSxTruthy((name == "case")) ? stepSfCase(args, env, kont) : (isSxTruthy((name == "and")) ? stepSfAnd(args, env, kont) : (isSxTruthy((name == "or")) ? stepSfOr(args, env, kont) : (isSxTruthy((name == "let")) ? stepSfLet(args, env, kont) : (isSxTruthy((name == "let*")) ? stepSfLet(args, env, kont) : (isSxTruthy((name == "lambda")) ? stepSfLambda(args, env, kont) : (isSxTruthy((name == "fn")) ? stepSfLambda(args, env, kont) : (isSxTruthy((name == "define")) ? stepSfDefine(args, env, kont) : (isSxTruthy((name == "defcomp")) ? makeCekValue(sfDefcomp(args, env), env, kont) : (isSxTruthy((name == "defisland")) ? makeCekValue(sfDefisland(args, env), env, kont) : (isSxTruthy((name == "defmacro")) ? makeCekValue(sfDefmacro(args, env), env, kont) : (isSxTruthy((name == "begin")) ? stepSfBegin(args, env, kont) : (isSxTruthy((name == "do")) ? stepSfBegin(args, env, kont) : (isSxTruthy((name == "quote")) ? makeCekValue((isSxTruthy(isEmpty(args)) ? NIL : first(args)), env, kont) : (isSxTruthy((name == "quasiquote")) ? makeCekValue(qqExpand(first(args), env), env, kont) : (isSxTruthy((name == "->")) ? stepSfThreadFirst(args, env, kont) : (isSxTruthy((name == "set!")) ? stepSfSet(args, env, kont) : (isSxTruthy((name == "letrec")) ? makeCekValue(sfLetrec(args, env), env, kont) : (isSxTruthy((name == "reset")) ? stepSfReset(args, env, kont) : (isSxTruthy((name == "shift")) ? stepSfShift(args, env, kont) : (isSxTruthy((name == "deref")) ? stepSfDeref(args, env, kont) : (isSxTruthy((name == "scope")) ? stepSfScope(args, env, kont) : (isSxTruthy((name == "provide")) ? stepSfProvide(args, env, kont) : (isSxTruthy((name == "context")) ? stepSfContext(args, env, kont) : (isSxTruthy((name == "emit!")) ? stepSfEmit(args, env, kont) : (isSxTruthy((name == "emitted")) ? stepSfEmitted(args, env, kont) : (isSxTruthy((name == "dynamic-wind")) ? makeCekValue(sfDynamicWind(args, env), env, kont) : (isSxTruthy((name == "map")) ? stepHoMap(args, env, kont) : (isSxTruthy((name == "map-indexed")) ? stepHoMapIndexed(args, env, kont) : (isSxTruthy((name == "filter")) ? stepHoFilter(args, env, kont) : (isSxTruthy((name == "reduce")) ? stepHoReduce(args, env, kont) : (isSxTruthy((name == "some")) ? stepHoSome(args, env, kont) : (isSxTruthy((name == "every?")) ? stepHoEvery(args, env, kont) : (isSxTruthy((name == "for-each")) ? stepHoForEach(args, env, kont) : (isSxTruthy(dictHas(_customSpecialForms, name)) ? makeCekValue([get(_customSpecialForms, name), args, env], env, kont) : (isSxTruthy((isSxTruthy(envHas(env, name)) && isMacro(envGet(env, name)))) ? (function() {
+  return (isSxTruthy((name == "if")) ? stepSfIf(args, env, kont) : (isSxTruthy((name == "when")) ? stepSfWhen(args, env, kont) : (isSxTruthy((name == "cond")) ? stepSfCond(args, env, kont) : (isSxTruthy((name == "case")) ? stepSfCase(args, env, kont) : (isSxTruthy((name == "and")) ? stepSfAnd(args, env, kont) : (isSxTruthy((name == "or")) ? stepSfOr(args, env, kont) : (isSxTruthy((name == "let")) ? stepSfLet(args, env, kont) : (isSxTruthy((name == "let*")) ? stepSfLet(args, env, kont) : (isSxTruthy((name == "lambda")) ? stepSfLambda(args, env, kont) : (isSxTruthy((name == "fn")) ? stepSfLambda(args, env, kont) : (isSxTruthy((name == "define")) ? stepSfDefine(args, env, kont) : (isSxTruthy((name == "defcomp")) ? makeCekValue(sfDefcomp(args, env), env, kont) : (isSxTruthy((name == "defisland")) ? makeCekValue(sfDefisland(args, env), env, kont) : (isSxTruthy((name == "defmacro")) ? makeCekValue(sfDefmacro(args, env), env, kont) : (isSxTruthy((name == "begin")) ? stepSfBegin(args, env, kont) : (isSxTruthy((name == "do")) ? stepSfBegin(args, env, kont) : (isSxTruthy((name == "quote")) ? makeCekValue((isSxTruthy(isEmpty(args)) ? NIL : first(args)), env, kont) : (isSxTruthy((name == "quasiquote")) ? makeCekValue(qqExpand(first(args), env), env, kont) : (isSxTruthy((name == "->")) ? stepSfThreadFirst(args, env, kont) : (isSxTruthy((name == "set!")) ? stepSfSet(args, env, kont) : (isSxTruthy((name == "letrec")) ? stepSfLetrec(args, env, kont) : (isSxTruthy((name == "reset")) ? stepSfReset(args, env, kont) : (isSxTruthy((name == "shift")) ? stepSfShift(args, env, kont) : (isSxTruthy((name == "deref")) ? stepSfDeref(args, env, kont) : (isSxTruthy((name == "scope")) ? stepSfScope(args, env, kont) : (isSxTruthy((name == "provide")) ? stepSfProvide(args, env, kont) : (isSxTruthy((name == "context")) ? stepSfContext(args, env, kont) : (isSxTruthy((name == "emit!")) ? stepSfEmit(args, env, kont) : (isSxTruthy((name == "emitted")) ? stepSfEmitted(args, env, kont) : (isSxTruthy((name == "dynamic-wind")) ? makeCekValue(sfDynamicWind(args, env), env, kont) : (isSxTruthy((name == "map")) ? stepHoMap(args, env, kont) : (isSxTruthy((name == "map-indexed")) ? stepHoMapIndexed(args, env, kont) : (isSxTruthy((name == "filter")) ? stepHoFilter(args, env, kont) : (isSxTruthy((name == "reduce")) ? stepHoReduce(args, env, kont) : (isSxTruthy((name == "some")) ? stepHoSome(args, env, kont) : (isSxTruthy((name == "every?")) ? stepHoEvery(args, env, kont) : (isSxTruthy((name == "for-each")) ? stepHoForEach(args, env, kont) : (isSxTruthy(dictHas(_customSpecialForms, name)) ? makeCekValue([get(_customSpecialForms, name), args, env], env, kont) : (isSxTruthy((isSxTruthy(envHas(env, name)) && isMacro(envGet(env, name)))) ? (function() {
   var mac = envGet(env, name);
   return makeCekState(expandMacro(mac, args, env), env, kont);
 })() : (isSxTruthy((isSxTruthy(_renderCheck) && _renderCheck(expr, env))) ? makeCekValue(_renderFn(expr, env), env, kont) : stepEvalCall(head, args, env, kont)))))))))))))))))))))))))))))))))))))))));