Add letrec to render-aware HTML forms — stepper island now SSRs

letrec in adapter-html.sx: evaluate via CEK (which handles mutual
recursion and returns a thunk), then render-value-to-html unwraps
the thunk and renders the expression with the letrec's local env.

Both islands (~layouts/header and ~home/stepper) now render
server-side with hydration markers and CSS classes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/adapter-html.sx

@@ -56,7 +56,7 @@
 ;; --------------------------------------------------------------------------
 
 (define RENDER_HTML_FORMS
-  (list "if" "when" "cond" "case" "let" "let*" "begin" "do"
+  (list "if" "when" "cond" "case" "let" "let*" "letrec" "begin" "do"
         "define" "defcomp" "defisland" "defmacro" "defstyle" "defhandler"
         "deftype" "defeffect"
         "map" "map-indexed" "filter" "for-each" "scope" "provide"))
@@ -173,6 +173,13 @@
       (= name "case")
         (render-to-html (trampoline (eval-expr expr env)) env)
 
+      ;; letrec — evaluate via CEK, render the result.
+      ;; sf-letrec returns a thunk; the thunk handler in render-value-to-html
+      ;; unwraps it and renders the expression with the letrec's local env.
+      (= name "letrec")
+        (let ((result (eval-expr expr env)))
+          (render-value-to-html result env))
+
       ;; let / let* — single body: pass through. Multi: join strings.
       (or (= name "let") (= name "let*"))
         (let ((local (process-bindings (nth expr 1) env)))