Stepper: steps-to-preview for isomorphic preview text (WIP)

steps-to-preview is a pure recursive descent function inside the island's
letrec that builds an SX expression tree from steps[0..target-1].
The preview lake uses it to show partial text (e.g. "the joy of " at step 9).

Still WIP: stepper island doesn't SSR because DOM-only code (effect,
dom-query, dom-create-element) runs in the island body and fails on server.
Need to guard client-only code so SSR can render the pure parts
(code view, counter, preview).

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 = "2026-03-24T04:10:36Z";
+  var SX_VERSION = "2026-03-24T04:23:51Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }