ocaml: phase 5.1 euler1.ml baseline (Project Euler #1, multiples of 3 or 5 below 1000 = 233168)

Project Euler #1: sum of all multiples of 3 or 5 below 1000.

  let euler1 limit =
    let sum = ref 0 in
    for i = 1 to limit - 1 do
      if i mod 3 = 0 || i mod 5 = 0 then sum := !sum + i
    done;
    !sum

  euler1 1000 = 233168

Trivial DSL exercise but symbolically meaningful: this is the 100th
baseline program.

100 baseline programs total — milestone.

plans/ocaml-on-sx.md

@@ -407,6 +407,9 @@ _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 — euler1.ml baseline (Project Euler #1, sum
+  of multiples of 3 or 5 below 1000 = 233168). Trivial DSL exercise
+  but symbolically meaningful: this is the 100th baseline program.
 - 2026-05-09 Phase 5.1 — anagram_groups.ml baseline (group strings
   by canonical anagram form, ["eat";"tea";"tan";"ate";"nat";"bat"]
   has 3 groups). canonical builds a sorted-by-frequency string