Add live CEK stepper island — interactive stepping debugger

A defisland that lets users type an SX expression, step through CEK
evaluation one transition at a time, and see C/E/K registers update
live. Demonstrates that cek-step is pure data->data.

- cek.sx geography: add ~geography/cek/demo-stepper island with
  source input, step/run/reset buttons, state display, step history
- platform_js.py: register CEK stepping primitives (make-cek-state,
  cek-step, cek-terminal?, cek-value, make-env, sx-serialize) so
  island code can access them

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 = "2026-03-14T10:44:25Z";
+  var SX_VERSION = "2026-03-14T15:23:04Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }
@@ -6494,6 +6494,8 @@ return (function() {
   if (typeof domOuterHtml === "function") PRIMITIVES["dom-outer-html"] = domOuterHtml;
   if (typeof domInnerHtml === "function") PRIMITIVES["dom-inner-html"] = domInnerHtml;
   if (typeof domTextContent === "function") PRIMITIVES["dom-text-content"] = domTextContent;
+  if (typeof domCreateElement === "function") PRIMITIVES["dom-create-element"] = domCreateElement;
+  if (typeof domAppend === "function") PRIMITIVES["dom-append"] = domAppend;
   if (typeof jsonParse === "function") PRIMITIVES["json-parse"] = jsonParse;
   if (typeof nowMs === "function") PRIMITIVES["now-ms"] = nowMs;
   PRIMITIVES["sx-parse"] = sxParse;
@@ -6548,6 +6550,7 @@ return (function() {
   };
 
 
+
   // =========================================================================
   // Async IO: Promise-aware rendering for client-side IO primitives
   // =========================================================================