ocaml: phase 5.1 lambda_calc.ml baseline (17/17 pass)

Untyped lambda calculus interpreter inside OCaml-on-SX:
  type term = Var | Abs of string * term | App | Num of int
  type value = VNum of int | VClos of string * term * env
  let rec eval env t = match t with ...

(\x.\y.x) 7 99 = 7. The substrate handles two ADTs, recursive eval,
closure-based env, and pattern matching all written as a single
self-contained OCaml program — strong validation.

plans/ocaml-on-sx.md

@@ -407,6 +407,12 @@ _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 — lambda_calc.ml baseline (17/17 pass). Untyped
+  lambda calculus interpreter using two ADTs (`type term = Var | Abs |
+  App | Num`, `type value = VNum | VClos`), an env as `(string * value)
+  list`, and recursive eval. `(\x.\y.x) 7 99 = 7` end-to-end. Demonstrates
+  the substrate handles a non-trivial AST + closure-based evaluator
+  written in OCaml-on-SX.
 - 2026-05-08 Phase 6 — Char predicates: is_digit/is_alpha/is_alnum/
   is_whitespace/is_upper/is_lower (+7 tests, 461 total). All written
   in OCaml in runtime.sx using Char.code + ASCII range checks.