ocaml: phase 5.1 roman.ml baseline + top-level 'let () = expr'

Side-quest emerged from adding roman.ml baseline (Roman numeral greedy
encoding): top-level 'let () = expr' was unsupported because
ocaml-parse-program's parse-decl-let consumed an ident strictly. Now
parse-decl-let recognises a leading '()' as a unit binding and
synthesises a __unit_NN name (matching how parse-let already handles
inner-let unit patterns).

roman.ml exercises:
  * tuple list literal [(int * string); ...]
  * recursive pattern match on tuple-cons
  * String.length + List.fold_left
  * the new top-level let () support (sanity in a comment, even though
    the program ends with a bare expression for the test harness)

Bumped lib/ocaml/test.sh server timeout 180->360s — the recent surge in
test count plus a CPU-contended host was crowding out the sole epoch
reaching the deeper smarts.

lib/ocaml/parser.sx

@@ -1147,7 +1147,16 @@
                 (when (at-kw? "rec") (begin (advance-tok!) (set! reccy true)))
                 (define parse-one!
                   (fn ()
-                    (let ((nm (ocaml-tok-value (consume! "ident" nil)))
+                    (let ((nm (cond
+                                ((and (at-op? "(")
+                                      (< (+ idx 1) tok-len)
+                                      (let ((t1 (nth tokens (+ idx 1))))
+                                        (and (= (ocaml-tok-type t1) "op")
+                                             (= (ocaml-tok-value t1) ")"))))
+                                  (begin (advance-tok!) (advance-tok!)
+                                         (str "__unit_" idx)))
+                                (else
+                                  (ocaml-tok-value (consume! "ident" nil)))))
                           (ps (list)))
                       (begin
                         (define collect-params