Server: render htmx (HX-Request) responses as HTML, not SX wire format

htmx sends HX-Request header on AJAX calls. The server now detects this
and renders the SX response to HTML via sx_render_to_html before sending.
SX-Request (from SX client navigation) still gets SX wire format.
Also skip response cache for htmx requests (they need fresh HTML renders).

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

hosts/ocaml/bin/sx_server.ml

@@ -3740,18 +3740,23 @@ let http_mode port =
     (try Unix.close client with _ -> ())
   in
 
-  (* Check if request has SX-Request or HX-Request header (AJAX navigation) *)
+  (* Check if request has SX-Request header (SX AJAX navigation — return SX wire format)
+     or HX-Request header (htmx AJAX — return rendered HTML). *)
+  let has_substring s sub =
+    let slen = String.length s and sublen = String.length sub in
+    if sublen > slen then false
+    else let rec check i = if i > slen - sublen then false
+      else if String.sub s i sublen = sub then true else check (i + 1)
+    in check 0
+  in
   let is_sx_request data =
     let lower = String.lowercase_ascii data in
-    let has_substring s sub =
-      let slen = String.length s and sublen = String.length sub in
-      if sublen > slen then false
-      else let rec check i = if i > slen - sublen then false
-        else if String.sub s i sublen = sub then true else check (i + 1)
-      in check 0
-    in
     has_substring lower "sx-request" || has_substring lower "hx-request"
   in
+  let _is_hx_request data =
+    let lower = String.lowercase_ascii data in
+    has_substring lower "hx-request" && not (has_substring lower "sx-request")
+  in
 
   (* Non-blocking event loop with render worker pool.
      - Main loop: Unix.select on listen socket + all connected clients
@@ -3785,14 +3790,23 @@ let http_mode port =
       match work with
       | Some (fd, path, headers) ->
         let is_ajax = headers <> [] in
-        let cache_key = if is_ajax then "ajax:" ^ path else path in
+        let is_htmx = List.exists (fun (k,_) -> String.lowercase_ascii k = "hx-request") headers in
+        let cache_key = if is_ajax then (if is_htmx then "htmx:" else "ajax:") ^ path else path in
         let response =
           try
             match http_render_page env path headers with
             | Some body ->
-              let ct = if is_ajax then "text/sx; charset=utf-8"
+              (* htmx requests get HTML; SX requests get SX wire format *)
+              let final_body = if is_htmx then
+                (try
+                  let exprs = Sx_parser.parse_all body in
+                  let expr = match exprs with [e] -> e | [] -> Nil | _ -> List (Symbol "<>" :: exprs) in
+                  Sx_render.sx_render_to_html env expr env
+                with _ -> body)
+              else body in
+              let ct = if is_ajax && not is_htmx then "text/sx; charset=utf-8"
                        else "text/html; charset=utf-8" in
-              let resp = http_response ~content_type:ct body in
+              let resp = http_response ~content_type:ct final_body in
               Hashtbl.replace response_cache cache_key resp;
               resp
             | None -> http_response ~status:404 "<h1>Not Found</h1>"
@@ -4120,20 +4134,35 @@ let http_mode port =
             end
           end else
           let has_state_cookie = Hashtbl.mem _request_cookies "sx-home-stepper" in
-          let cache_key = if is_ajax then "ajax:" ^ path else path in
-          match (if has_state_cookie then None
+          let is_htmx_req = is_ajax && has_substring (String.lowercase_ascii data) "hx-request" in
+          let cache_key = if is_htmx_req then "htmx:" ^ path
+                          else if is_ajax then "ajax:" ^ path else path in
+          match (if has_state_cookie || is_htmx_req then None
                  else Hashtbl.find_opt response_cache cache_key) with
           | Some cached -> write_response fd cached; true
           | None ->
             if is_ajax then begin
               (* AJAX: render on main thread — aser only, fast, no SSR.
-                 Avoids queueing behind slow full-page renders. *)
+                 Avoids queueing behind slow full-page renders.
+                 HX-Request (htmx) gets HTML; SX-Request gets SX wire format. *)
               let headers = parse_http_headers data in
+              let is_htmx = List.exists (fun (k,_) ->
+                String.lowercase_ascii k = "hx-request") headers in
               let response =
                 try match http_render_page env path headers with
                   | Some body ->
-                    let resp = http_response ~content_type:"text/sx; charset=utf-8" body in
-                    Hashtbl.replace response_cache cache_key resp; resp
+                    let final_body = if is_htmx then
+                      (try
+                        let exprs = Sx_parser.parse_all body in
+                        let expr = match exprs with [e] -> e | [] -> Nil | _ -> List (Symbol "<>" :: exprs) in
+                        Sx_render.sx_render_to_html env expr env
+                      with _ -> body)
+                    else body in
+                    let ct = if is_htmx then "text/html; charset=utf-8"
+                             else "text/sx; charset=utf-8" in
+                    let resp = http_response ~content_type:ct final_body in
+                    if not is_htmx then Hashtbl.replace response_cache cache_key resp;
+                    resp
                   | None -> http_response ~status:404
                     "(div :class \"p-8\" (h2 :class \"text-rose-600 font-semibold\" \"Page not found\") (p :class \"text-stone-500\" \"No route matched this path\"))"
                 with e ->