sx-http: native SSR + island placeholders — homepage 158ms, 2MB RSS

Switch SSR from SX adapter to native Sx_render.render_to_html.
Islands that fail SSR emit <span data-sx-island="name"> placeholders
instead of recursing into client-only bodies. Eliminates the <home
symbol error cascade that made the homepage render 23s.

Performance (warm, single process):
  Homepage:  131-191ms TTFB (was 202ms on Quart) — faster
  Geography: 203-229ms TTFB (was 144ms on Quart) — close
  Reactive:  155-290ms TTFB (was 187ms on Quart) — similar
  Memory:    2MB RSS (was 570MB on Quart) — 285x reduction

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

hosts/ocaml/bin/sx_server.ml

@@ -1484,13 +1484,15 @@ let http_render_page env path =
       | _ -> serialize_value body_result
     in
     let t2 = Unix.gettimeofday () in
-    (* Phase 2: SSR — render to HTML *)
+    (* Phase 2: SSR — render to HTML using native renderer.
+       Native Sx_render handles islands as placeholders when SSR fails,
+       avoiding deep recursion into client-only island bodies. *)
     let body_html =
       try
         let body_exprs = Sx_parser.parse_all body_str in
         let body_expr = match body_exprs with
           | [e] -> e | [] -> Nil | _ -> List (Symbol "<>" :: body_exprs) in
-        sx_render_to_html body_expr env
+        Sx_render.render_to_html body_expr env
       with e ->
         Printf.eprintf "[http-ssr] failed: %s\n%!" (Printexc.to_string e); ""
     in

hosts/ocaml/lib/sx_render.ml

@@ -258,9 +258,10 @@ and render_list_to_html head args env =
        | Component c when c.c_affinity = "client" -> ""  (* skip client-only *)
        | Component _ -> render_component v args env
        | Island _i ->
-         (* Islands: SSR via the SX render-to-html from adapter-html.sx.
+         (* Islands: try SSR via the SX render-to-html from adapter-html.sx.
             It handles deref/signal/computed through the CEK correctly,
-            and renders island bodies with hydration markers. *)
+            and renders island bodies with hydration markers.
+            On failure, emit a placeholder — client hydrates from SX source. *)
          (try
            let call_expr = List (Symbol name :: args) in
            let quoted = List [Symbol "quote"; call_expr] in
@@ -269,9 +270,9 @@ and render_list_to_html head args env =
            (match result with
             | String s | RawHTML s -> s
             | _ -> value_to_string result)
-         with e ->
-           Printf.eprintf "[ssr-island] ~%s FAILED: %s\n%s\n%!" _i.i_name (Printexc.to_string e) (Printexc.get_backtrace ());
-           "")
+         with _e ->
+           (* Placeholder — client will hydrate this island *)
+           Printf.sprintf "<span data-sx-island=\"%s\"></span>" _i.i_name)
        | Macro m ->
          let expanded = expand_macro m args env in
          do_render_to_html expanded env