ocaml: phase 5.1 euler34_small.ml baseline (factorions <= 2000 = 145)

Number that equals the sum of factorials of its digits:
  145 = 1! + 4! + 5! = 1 + 24 + 120

Implementation:
  fact n           iterative factorial
  digit_fact_sum n walk digits, sum fact(digit)
  euler34 limit    scan 3..limit, accumulate matches

The only other factorion is 40585 = 4!+0!+5!+8!+5!. Real PE34 sums
both (= 40730); 2000 keeps under our search budget.

121 baseline programs total.

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 — euler34_small.ml baseline (numbers equal
+  to sum of factorials of digits, ≤2000 = 145). 145 = 1!+4!+5! =
+  1+24+120. The other "factorion" 40585 is the only number above
+  this threshold; PE34 sums both = 40730. 121 baseline programs
+  total.
 - 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