ocaml: phase 5.1 zip_unzip.ml baseline (zip/unzip round-trip, sum-product = 1000)

zip walks both lists in lockstep, truncating at the shorter. unzip
uses tuple-pattern destructuring on the recursive result.

  let pairs = zip [1;2;3;4] [10;20;30;40] in
  let (xs, ys) = unzip pairs in
  List.fold_left (+) 0 xs * List.fold_left (+) 0 ys
  = 10 * 100
  = 1000

Exercises:
  - tuple-cons patterns in match scrutinee: 'match (xs, ys) with'
  - tuple constructor in return value: '(a :: la, b :: lb)'
  - the iter-98 let-tuple destructuring: 'let (la, lb) = unzip rest'

53 baseline programs total.

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.
+- 2026-05-09 Phase 5.1 — zip_unzip.ml baseline (list zip/unzip
+  round-trip, sum-product = 1000). zip walks both lists in lockstep
+  truncating at the shorter; unzip uses tuple-pattern destructuring
+  on the recursive result. After zip [1;2;3;4] [10;20;30;40] +
+  unzip, sums are 10 and 100 → product 1000. Exercises tuple-cons
+  patterns in match scrutinee `(xs, ys)`, tuple constructor in
+  return value `(a :: la, b :: lb)`, and the iter-98 let-tuple
+  destructuring `let (la, lb) = unzip rest in`. 53 baseline programs
+  total.
 - 2026-05-09 Phase 5.1 — bigint_add.ml baseline (digit-list big-num
   add, 28 = 1+18+9). Recursive 4-arm match on `(a, b)` tuples
   threading a carry: `(x::xs, y::ys) -> (s mod 10) :: aux xs ys (s