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>

hosts/javascript/platform.py

@@ -2027,8 +2027,8 @@ PLATFORM_DOM_JS = """
     // If lambda takes 0 params, call without event arg (convenience for on-click handlers)
     var wrapped = isLambda(handler)
       ? (lambdaParams(handler).length === 0
-        ? function(e) { try { cekCall(handler, NIL); } catch(err) { console.error("[sx-ref] domListen handler error:", name, err); } finally { runPostRenderHooks(); } }
-        : function(e) { try { cekCall(handler, [e]); } catch(err) { console.error("[sx-ref] domListen handler error:", name, err); } finally { runPostRenderHooks(); } })
+        ? function(e) { try { cekCall(handler, NIL); } catch(err) { console.error("[sx-ref] domListen handler error:", name, err); } }
+        : function(e) { try { cekCall(handler, [e]); } catch(err) { console.error("[sx-ref] domListen handler error:", name, err); } })
       : handler;
     if (name === "click") logInfo("domListen: click on <" + (el.tagName||"?").toLowerCase() + "> text=" + (el.textContent||"").substring(0,20) + " isLambda=" + isLambda(handler));
     var passiveEvents = { touchstart: 1, touchmove: 1, wheel: 1, scroll: 1 };