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/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: