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