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>

hosts/ocaml/bin/sx_server.ml

@@ -1761,6 +1761,9 @@ let http_inject_shell_statics env static_dir sx_sxc =
   let templates_dir = project_dir ^ "/shared/sx/templates" in
   let client_libs = [
     templates_dir ^ "/cssx.sx";
+    templates_dir ^ "/tw-layout.sx";
+    templates_dir ^ "/tw-type.sx";
+    templates_dir ^ "/tw.sx";
   ] in
   List.iter (fun path ->
     if Sys.file_exists path then begin
@@ -2593,6 +2596,21 @@ let http_mode port =
         _req_headers := parse_http_headers data;
         _req_body := (if method_ = "POST" || method_ = "PUT" || method_ = "PATCH"
                       then extract_body data else "");
+        (* Parse Cookie header into request_cookies for get-cookie primitive *)
+        Hashtbl.clear _request_cookies;
+        (match List.assoc_opt "cookie"
+           (List.map (fun (k,v) -> (String.lowercase_ascii k, v)) !_req_headers) with
+         | Some cookie_str ->
+           List.iter (fun pair ->
+             let trimmed = String.trim pair in
+             (match String.index_opt trimmed '=' with
+              | Some i ->
+                let k = String.sub trimmed 0 i in
+                let v = String.sub trimmed (i+1) (String.length trimmed - i - 1) in
+                Hashtbl.replace _request_cookies k v
+              | None -> ())
+           ) (String.split_on_char ';' cookie_str)
+         | None -> ());
         if path = "/" then begin
           write_response fd (http_redirect "/sx/"); true
         end else
@@ -2736,7 +2754,10 @@ let http_mode port =
           in
           write_response fd response; true
         end else if is_sx then begin
-          let cache_key = if is_ajax then "ajax:" ^ path else path in
+          let has_stepper_cookie = Hashtbl.mem _request_cookies "sx-home-stepper" in
+          let cache_key = if is_ajax then "ajax:" ^ path
+            else if has_stepper_cookie then path ^ ":step=" ^ (try Hashtbl.find _request_cookies "sx-home-stepper" with Not_found -> "")
+            else path in
           match Hashtbl.find_opt response_cache cache_key with
           | Some cached -> write_response fd cached; true
           | None ->

hosts/ocaml/browser/sx_browser.ml

@@ -401,6 +401,7 @@ let api_register_native name_js callback_js =
     js_to_value (Js.Unsafe.fun_call callback_js [| Js.Unsafe.inject (Js.array js_args) |])
   in
   let v = NativeFn (name, native_fn) in
+  Sx_primitives.register name native_fn;
   ignore (env_bind global_env name v);
   Hashtbl.replace _vm_globals name v;
   Js.Unsafe.inject Js.null