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 +1508,21 @@ CEK_FIXUPS_JS = '''
     while (!cekTerminal_p(state)) { state = cekStep(state); }
     return cekValue(state);
   };
+
+  // CEK stepping primitives — for debugger islands
+  PRIMITIVES["make-cek-state"] = makeCekState;
+  PRIMITIVES["cek-step"] = cekStep;
+  PRIMITIVES["cek-terminal?"] = cekTerminal_p;
+  PRIMITIVES["cek-value"] = cekValue;
+  PRIMITIVES["make-env"] = function() { return merge(PRIMITIVES); };
+  PRIMITIVES["sx-serialize"] = sxSerialize;
+  PRIMITIVES["lambda-name"] = lambdaName;
+  PRIMITIVES["callable?"] = isCallable;
+  PRIMITIVES["is-html-tag?"] = function(name) { return HTML_TAGS.indexOf(name) >= 0; };
+  PRIMITIVES["make-symbol"] = function(name) { return new Symbol(name); };
+  PRIMITIVES["keyword-name"] = keywordName;
+  PRIMITIVES["type-of"] = typeOf;
+  PRIMITIVES["symbol-name"] = symbolName;
 '''