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/sx/ref/platform_js.py

@@ -1508,6 +1508,35 @@ CEK_FIXUPS_JS = '''
     while (!cekTerminal_p(state)) { state = cekStep(state); }
     return cekValue(state);
   };
+
+  // Expose spec functions so evaluated SX code can use them.
+  // Type inspection (platform interface from boundary.sx)
+  PRIMITIVES["type-of"] = typeOf;
+  PRIMITIVES["symbol-name"] = symbolName;
+  PRIMITIVES["keyword-name"] = keywordName;
+  PRIMITIVES["callable?"] = isCallable;
+  PRIMITIVES["lambda?"] = isLambda;
+  PRIMITIVES["lambda-name"] = lambdaName;
+  PRIMITIVES["component?"] = isComponent;
+  PRIMITIVES["island?"] = isIsland;
+  PRIMITIVES["make-symbol"] = function(n) { return new Symbol(n); };
+
+  // Parser (from parser.sx)
+  PRIMITIVES["sx-serialize"] = sxSerialize;
+
+  // CEK machine (from cek.sx/frames.sx)
+  PRIMITIVES["make-cek-state"] = makeCekState;
+  PRIMITIVES["cek-step"] = cekStep;
+  PRIMITIVES["cek-run"] = cekRun;
+  PRIMITIVES["cek-terminal?"] = cekTerminal_p;
+  PRIMITIVES["cek-value"] = cekValue;
+  PRIMITIVES["eval-expr-cek"] = evalExprCek;
+
+  // Render (from adapter-html.sx / render.sx)
+  PRIMITIVES["is-html-tag?"] = function(n) { return HTML_TAGS.indexOf(n) >= 0; };
+
+  // Environment (for creating eval contexts)
+  PRIMITIVES["make-env"] = function() { return merge(componentEnv, PRIMITIVES); };
 '''
 
 
@@ -3033,6 +3062,8 @@ def fixups_js(has_html, has_sx, has_dom, has_signals=False, has_deps=False, has_
   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;

shared/sx/ref/run_js_sx.py

@@ -216,13 +216,15 @@ def compile_ref_to_js(
     # Platform JS for selected adapters
     if not has_dom:
         parts.append("\n  var _hasDom = false;\n")
-    for name in ("dom", "engine", "orchestration", "boot"):
-        if name in adapter_set and name in adapter_platform:
-            parts.append(adapter_platform[name])
 
+    # CEK fixups + general fixups BEFORE boot (boot hydrates islands that need these)
     parts.append(fixups_js(has_html, has_sx, has_dom, has_signals, has_deps, has_page_helpers))
     if has_cek:
         parts.append(CEK_FIXUPS_JS)
+
+    for name in ("dom", "engine", "orchestration", "boot"):
+        if name in adapter_set and name in adapter_platform:
+            parts.append(adapter_platform[name])
     if has_continuations:
         parts.append(CONTINUATIONS_JS)
     if has_dom: