ocaml: phase 5.1 expr_eval.ml baseline (9/9 pass)

A tiny arithmetic-expression evaluator using:
  type expr = Lit of int | Add of expr*expr | Mul of expr*expr | Neg of expr
  let rec eval e = match e with | Lit n -> n | Add (a,b) -> ...

Exercises type-decl + multi-arg ctor + recursive match end-to-end.
Per-program timeout in run.sh bumped to 120s.

plans/ocaml-on-sx.md

@@ -377,6 +377,11 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 5.1 — expr_eval.ml baseline (9/9 pass). A tiny
+  arithmetic-expression evaluator using ADT (`type expr = Lit | Add |
+  Mul | Neg`) + recursive eval + pattern match — exercises the full
+  type-decl + ctor + match pipeline end-to-end. Per-program timeout
+  bumped to 120s in run.sh.
 - 2026-05-08 Phase 3 — polymorphic variants `` `Tag `` (+4 tests, 382
   total). Tokenizer recognises backtick followed by an upper ident,
   tokenizing identically to nominal ctors. Parser and evaluator treat