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.

plans/ocaml-on-sx.md

@@ -407,6 +407,15 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-08 Phase 5.1 — roman.ml baseline (Roman numeral greedy
+  encoding). Side-quest: top-level `let () = expr` was unsupported by
+  ocaml-parse-program — now parse-decl-let recognises `()` as a unit
+  binding (`__unit_NN` synthetic name), matching the inner-let handling
+  in parse-let. roman.ml uses recursive pattern match on
+  `(int * string) list` greedy table + `List.fold_left + String.length`
+  to compute the cumulative length of 9 encoded numbers (44).
+  Bumped test.sh server timeout 180→360s for headroom on contended
+  systems.
 - 2026-05-08 Phase 4 — `M.(expr)` local-open expression form (+3
   tests, 481 total). Implemented in parse-atom-postfix: after
   consuming `.`, if next token is `(`, parse the inner expression and