Stepper cookie persistence: SSR + client-side save/restore

- Parse Cookie header in OCaml HTTP server for get-cookie primitive
- Stepper saves step-idx to cookie via host-set! FFI on click
- Stepper restores from cookie: get-cookie on server, host-get FFI on client
- Cache key includes stepper cookie value to avoid stale SSR
- registerNative: also update Sx_primitives table for CALL_PRIM dispatch

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

sx/sx/home-stepper.sx

@@ -2,9 +2,10 @@
   ~home/stepper
   ()
   (let
-    ((source "(div (~cssx/tw :tokens \"text-center\")\n  (h1 (~cssx/tw :tokens \"text-3xl font-bold mb-2\")\n    (span (~cssx/tw :tokens \"text-rose-500\") \"the \")\n    (span (~cssx/tw :tokens \"text-amber-500\") \"joy \")\n    (span (~cssx/tw :tokens \"text-emerald-500\") \"of \")\n    (span (~cssx/tw :tokens \"text-violet-600 text-4xl\") \"sx\")))")
+    ((source "(div (~tw :tokens \"text-center\")\n  (h1 (~tw :tokens \"text-3xl font-bold mb-2\")\n    (span (~tw :tokens \"text-rose-500\") \"the \")\n    (span (~tw :tokens \"text-amber-500\") \"joy \")\n    (span (~tw :tokens \"text-emerald-500\") \"of \")\n    (span (~tw :tokens \"text-violet-600 text-4xl\") \"sx\")))")
       (steps (signal (list)))
-      (store (if (client?) (def-store "home-stepper" (fn () {:step-idx (signal 9)})) nil))
+      (store
+        (if (client?) (def-store "home-stepper" (fn () {:step-idx (signal 9)})) nil))
       (step-idx (if store (get store "step-idx") (signal 9)))
       (dom-stack-sig (signal (list)))
       (code-tokens (signal (list))))
@@ -204,7 +205,7 @@
                             (= (keyword-name (nth sp 1)) "tokens")
                             (string? (nth sp 2)))
                           (let
-                            ((result (trampoline (~cssx/tw :tokens (nth sp 2)))))
+                            ((result (trampoline (~tw :tokens (nth sp 2)))))
                             (when
                               (spread? result)
                               (let
@@ -239,7 +240,14 @@
                   (= step-type "expr")
                   nil))
               (swap! step-idx inc)
-              (update-code-highlight))))
+              (update-code-highlight)
+              (host-set!
+                (dom-document)
+                "cookie"
+                (str
+                  "sx-home-stepper="
+                  (deref step-idx)
+                  ";path=/;max-age=31536000;SameSite=Lax")))))
         (rebuild-preview
           (fn
             (target)
@@ -266,16 +274,22 @@
                 (rebuild-preview target)
                 (reset! step-idx target)
                 (update-code-highlight)
-)))))
-
+                (host-set!
+                  (dom-document)
+                  "cookie"
+                  (str
+                    "sx-home-stepper="
+                    target
+                    ";path=/;max-age=31536000;SameSite=Lax")))))))
       (let
-        ((saved (get-cookie "sx-home-stepper")))
+        ((saved (if (client?) (let ((raw (host-get (dom-document) "cookie")) (prefix "sx-home-stepper=")) (when (and (string? raw) (contains? raw prefix)) (let ((start (+ (index-of raw prefix) (len prefix)))) (let ((rest (slice raw start)) (end-pos (index-of rest ";"))) (if (> end-pos -1) (slice rest 0 end-pos) rest))))) (get-cookie "sx-home-stepper"))))
         (when
-          saved
-          (thaw-from-sx saved)
-          (when
-            (or (< (deref step-idx) 0) (> (deref step-idx) 16))
-            (reset! step-idx 9))))
+          (string? saved)
+          (let
+            ((n (parse-number saved)))
+            (when
+              (and (number? n) (>= n 0) (<= n 16))
+              (reset! step-idx n)))))
       (let
         ((parsed (sx-parse source)))
         (when
@@ -292,10 +306,10 @@
       (let
         ((_eff (effect (fn () (schedule-idle (fn () (build-code-dom) (rebuild-preview (deref step-idx)) (update-code-highlight) (run-post-render-hooks)))))))
         (div
-          :class "space-y-4"
+          (~tw :tokens "space-y-4 text-center")
           (div
             :data-code-view true
-            (~cssx/tw
+            (~tw
               :tokens "font-mono bg-stone-50 rounded p-2 overflow-x-auto leading-relaxed whitespace-pre-wrap")
             :style "font-size:0.5rem"
             (map
@@ -319,12 +333,9 @@
                   (span :class cls (get tok "text"))))
               (deref code-tokens)))
           (div
-            :class "flex items-center justify-center gap-2 md:gap-3"
+            (~tw :tokens "flex items-center justify-center gap-2 md:gap-3")
             (button
-              :on-click (fn
-                (e)
-                (do-back)
-)
+              :on-click (fn (e) (do-back))
               :class (str
                 "px-2 py-1 rounded text-3xl "
                 (if
@@ -333,15 +344,12 @@
                   "text-stone-300 cursor-not-allowed"))
               "◀")
             (span
-              :class "text-sm text-stone-500 font-mono tabular-nums"
+              (~tw :tokens "text-sm text-stone-500 font-mono tabular-nums")
               (deref step-idx)
               " / "
               (len (deref steps)))
             (button
-              :on-click (fn
-                (e)
-                (do-step)
-)
+              :on-click (fn (e) (do-step))
               :class (str
                 "px-2 py-1 rounded text-3xl "
                 (if