Fix stepper client-side [object Object] flash and missing CSSX styles

Three issues in the stepper island's client-side rendering:

1. do-step used eval-expr with empty env for ~cssx/tw spreads — component
   not found, result leaked as [object Object]. Fixed: call ~cssx/tw
   directly (in scope from island env) with trampoline.

2. steps-to-preview excluded spreads — SSR preview had no styling.
   Fixed: include spreads in the tree so both SSR and client render
   with CSSX classes.

3. build-children used named let (let loop ...) which produces
   unresolved Thunks in render mode due to the named-let compiler
   desugaring interacting with the render/eval boundary. Fixed:
   rewrote as plain recursive function bc-loop avoiding named let.

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 = "2026-03-24T22:56:21Z";
+  var SX_VERSION = "2026-03-25T00:09:53Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }