ocaml: phase 5.1 euler29_small.ml baseline (distinct a^b for 2<=a,b<=5 = 15)

Compute every power a^b for a, b in [2..N] and count distinct
values. Hashtbl as a set with unit-payload (iter-168 idiom):

  let euler29 n =
    let h = Hashtbl.create 64 in
    for a = 2 to n do
      for b = 2 to n do
        let p = ref 1 in
        for _ = 1 to b do p := !p * a done;
        Hashtbl.replace h !p ()
      done
    done;
    Hashtbl.length h

For N=5: 16 powers minus one duplicate (4^2 = 2^4 = 16) -> 15.

Real PE29 uses N=100 (answer 9183).

120 baseline programs total — milestone.

plans/ocaml-on-sx.md

@@ -407,6 +407,11 @@ _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 — euler29_small.ml baseline (distinct a^b for
+  2≤a,b≤5 = 15). 16 powers minus the one duplicate (4^2 = 2^4 = 16)
+  → 15 distinct values. Hashtbl as a set with unit-payload (the
+  iter-168 idiom). Real PE29 uses N=100 (answer 9183). 120 baseline
+  programs total — milestone.
 - 2026-05-09 Phase 5.1 — euler21_small.ml baseline (sum of amicable
   numbers ≤ 300 = 504). div_sum computes proper divisor sum via
   trial division up to √n; outer loop finds amicable pairs (a, b)