ocaml: phase 5.1 expr_simp.ml baseline (symbolic simplifier, eval(simp e) = 22)

Recursive ADT with three constructors (Num/Add/Mul). simp does
bottom-up rewrite using algebraic identities:

  x + 0  -> x
  0 + x  -> x
  x * 0  -> 0
  0 * x  -> 0
  x * 1  -> x
  1 * x  -> x
  constant folding for Num + Num and Num * Num

Uses tuple pattern in nested match: 'match (simp a, simp b) with'.

  Add (Mul (Num 3, Num 5), Add (Num 0, Mul (Num 1, Num 7)))
   -> simp ->  Add (Num 15, Num 7)
   -> eval ->  22

51 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,14 @@ _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.
+- 2026-05-09 Phase 5.1 — expr_simp.ml baseline (symbolic expression
+  simplifier, eval (simp e) = 22). Recursive ADT with three
+  constructors (Num/Add/Mul). simp does bottom-up rewrite using
+  algebraic identities: x+0 → x, 0+x → x, x*0 → 0, 0*x → 0, x*1 → x,
+  1*x → x, constant folding both. Uses tuple pattern in nested match
+  (`match (simp a, simp b) with`). For `Add (Mul (Num 3, Num 5),
+  Add (Num 0, Mul (Num 1, Num 7)))` → simp → `Add (Num 15, Num 7)`
+  → eval → 22. 51 baseline programs total.
 - 2026-05-09 Phase 5.1 — mat_mul.ml baseline (3x3 row-major matrix
   multiply, sum of result = 621). Triple-nested for loop over
   i / j / k with row-major indexing `c.(i * n + j) <- c.(i * n + j) +