spec: gensym + symbol interning (OCaml + tests)

gensym_counter ref + gensym/string->symbol/symbol->string/intern/symbol-interned?
primitives in sx_primitives.ml. Fix ListRef case in seq_to_list on both
sx_ref.ml and sx_primitives.ml. 19 new tests in test-gensym.sx.
OCaml 4450/1080, JS 2205/2497, zero regressions.

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

hosts/ocaml/lib/sx_ref.ml

@@ -29,6 +29,7 @@ let seq_to_list v =
   match v with
   | Nil -> List []
   | List _ -> v
+  | ListRef { contents = items } -> List items
   | Vector arr -> List (Array.to_list arr)
   | String s ->
     let chars = ref [] in