OCaml raw! in HTML renderer + SX_USE_OCAML env promotion + golden tests

- sx_render.ml: add raw! handler to HTML renderer (inject pre-rendered
  content without HTML escaping)
- docker-compose.yml: move SX_USE_OCAML/SX_OCAML_BIN to shared env
  (available to all services, not just sx_docs)
- hosts/ocaml/Dockerfile: OCaml kernel build stage
- shared/sx/tests/: golden test data + generator for OCaml render tests

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

hosts/ocaml/lib/sx_render.ml

@@ -205,6 +205,12 @@ and render_list_to_html head args env =
   match head with
   | Symbol "<>" ->
     render_children args env
+  | Symbol "raw!" ->
+    (* Inject pre-rendered HTML without escaping *)
+    let v = Sx_ref.eval_expr (List.hd args) (Env env) in
+    (match v with
+     | String s | RawHTML s -> s
+     | _ -> value_to_string v)
   | Symbol tag when is_html_tag tag ->
     render_html_element tag args env
   | Symbol "if" ->