ocaml: string ports (Eof + Port variants, 15 primitives)

Eof sentinel and Port{PortInput/PortOutput} in sx_types.ml. All 15 port
primitives in sx_primitives.ml. type_of/inspect updated. 39/39 port tests
pass (4532 total, +39, zero regressions).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hosts/ocaml/bin/sx_server.ml

@@ -1392,6 +1392,8 @@ let rec dispatch env cmd =
         | SxExpr s -> s
         | RawHTML s -> "\"" ^ escape_sx_string s ^ "\""
         | Char n -> Sx_types.inspect (Char n)
+        | Eof -> Sx_types.inspect Eof
+        | Port _ -> Sx_types.inspect result
         | _ -> "nil"
       in
       send_ok_raw (raw_serialize result)