Add index-of string primitive: spec, Python, JS, rebootstrap

(index-of s needle from?) returns first index of needle in s, or -1.
Optional start offset. Specced in primitives.sx, implemented in both
hand-written primitives.py and bootstrapper templates, rebootstrapped.

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

shared/static/scripts/sx-ref.js

@@ -283,6 +283,7 @@
   PRIMITIVES["split"] = function(s, sep) { return String(s).split(sep || " "); };
   PRIMITIVES["join"] = function(sep, coll) { return coll.join(sep); };
   PRIMITIVES["replace"] = function(s, old, nw) { return s.split(old).join(nw); };
+  PRIMITIVES["index-of"] = function(s, needle, from) { return String(s).indexOf(needle, from || 0); };
   PRIMITIVES["starts-with?"] = function(s, p) { return String(s).indexOf(p) === 0; };
   PRIMITIVES["ends-with?"] = function(s, p) { var str = String(s); return str.indexOf(p, str.length - p.length) !== -1; };
   PRIMITIVES["slice"] = function(c, a, b) { return b !== undefined ? c.slice(a, b) : c.slice(a); };
@@ -3316,87 +3317,6 @@ callExpr.push(dictGet(kwargs, k)); } }
   if (typeof aser === "function") PRIMITIVES["aser"] = aser;
   if (typeof renderToDom === "function") PRIMITIVES["render-to-dom"] = renderToDom;
 
-  // =========================================================================
-  // Extension: Delimited continuations (shift/reset)
-  // =========================================================================
-
-  function Continuation(fn) { this.fn = fn; }
-  Continuation.prototype._continuation = true;
-  Continuation.prototype.call = function(value) { return this.fn(value !== undefined ? value : NIL); };
-
-  function ShiftSignal(kName, body, env) {
-    this.kName = kName;
-    this.body = body;
-    this.env = env;
-  }
-
-  PRIMITIVES["continuation?"] = function(x) { return x != null && x._continuation === true; };
-
-  var _resetResume = [];
-
-  function sfReset(args, env) {
-    var body = args[0];
-    try {
-      return trampoline(evalExpr(body, env));
-    } catch (e) {
-      if (e instanceof ShiftSignal) {
-        var sig = e;
-        var cont = new Continuation(function(value) {
-          if (value === undefined) value = NIL;
-          _resetResume.push(value);
-          try {
-            return trampoline(evalExpr(body, env));
-          } finally {
-            _resetResume.pop();
-          }
-        });
-        var sigEnv = merge(sig.env);
-        sigEnv[sig.kName] = cont;
-        return trampoline(evalExpr(sig.body, sigEnv));
-      }
-      throw e;
-    }
-  }
-
-  function sfShift(args, env) {
-    if (_resetResume.length > 0) {
-      return _resetResume[_resetResume.length - 1];
-    }
-    var kName = symbolName(args[0]);
-    var body = args[1];
-    throw new ShiftSignal(kName, body, env);
-  }
-
-  // Wrap evalList to intercept reset/shift
-  var _baseEvalList = evalList;
-  evalList = function(expr, env) {
-    var head = expr[0];
-    if (isSym(head)) {
-      var name = head.name;
-      if (name === "reset") return sfReset(expr.slice(1), env);
-      if (name === "shift") return sfShift(expr.slice(1), env);
-    }
-    return _baseEvalList(expr, env);
-  };
-
-  // Wrap aserSpecial to handle reset/shift in SX wire mode
-  if (typeof aserSpecial === "function") {
-    var _baseAserSpecial = aserSpecial;
-    aserSpecial = function(name, expr, env) {
-      if (name === "reset") return sfReset(expr.slice(1), env);
-      if (name === "shift") return sfShift(expr.slice(1), env);
-      return _baseAserSpecial(name, expr, env);
-    };
-  }
-
-  // Wrap typeOf to recognize continuations
-  var _baseTypeOf = typeOf;
-  typeOf = function(x) {
-    if (x != null && x._continuation) return "continuation";
-    return _baseTypeOf(x);
-  };
-
-
   // Parser — compiled from parser.sx (see PLATFORM_PARSER_JS for ident char classes)
   var parse = sxParse;
 
@@ -3488,4 +3408,4 @@ callExpr.push(dictGet(kwargs, k)); } }
   if (typeof module !== "undefined" && module.exports) module.exports = Sx;
   else global.Sx = Sx;
 
-})(typeof globalThis !== "undefined" ? globalThis : typeof window !== "undefined" ? window : this);
+})(typeof globalThis !== "undefined" ? globalThis : typeof window !== "undefined" ? window : this);