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>

sx/sx/page-functions.sx

@@ -461,6 +461,13 @@
 (define protocol
   (make-page-fn "~protocols/wire-format-content" "~protocols/" nil "-content"))
 
+;; sx-pub (under applications)
+(define sx-pub
+  (fn (slug)
+    (if (nil? slug)
+      '(~sx-pub/overview-content)
+      nil)))
+
 ;; Essays (under etc)
 ;; Convention: ~essays/{slug}/essay-{slug}
 (define essay