ocaml: phase 5.1 zerosafe.ml baseline (Option-chained safe division, sum = 28)

safe_div returns None on division by zero; safe_chain stitches two
divisions, propagating None on either failure:

  let safe_div a b =
    if b = 0 then None else Some (a / b)

  let safe_chain a b c =
    match safe_div a b with
    | None -> None
    | Some q -> safe_div q c

Test:
  safe_chain 100 2 5    = Some 10
  safe_chain 100 0 5    = None  -> -1
  safe_chain 50 5 0     = None  -> -1
  safe_chain 1000 10 5  = Some 20
  10 - 1 - 1 + 20 = 28

Tests Option chaining + match-on-result with sentinel default.
Demonstrates the canonical 'fail-early on None' pattern.

129 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.
+- 2026-05-10 Phase 5.1 — zerosafe.ml baseline (Option-chained safe
+  division, sum 10 + (-1) + (-1) + 20 = 28). safe_div returns None
+  on division by zero; safe_chain stitches two divisions, propagating
+  None on either failure. Tests Option chaining + match-on-result
+  with sentinel default. 129 baseline programs total.
 - 2026-05-10 Phase 5.1 — number_words.ml baseline (letter count of
   numbers 1-19 spelled out = 106). 19-arm match dispatch returning
   the English word for each number. Sums lengths over 1..19. Real