ocaml: phase 5.1 safe_div.ml baseline + Result.equal/compare/iter_error (+3 tests, 592 total)

safe_div.ml: integer division returning Result. Sum-safe folds pairs,
skipping the Error branches.

  [(10,2); (20,4); (30,0); (50,5)]   ->  5 + 5 + 0 + 10 = 20

Result module additions (mirroring real OCaml's signatures):

  equal eq_ok eq_err a b
  compare cmp_ok cmp_err a b      Ok < Error (i.e. Ok x compared to
                                  Error e returns -1)
  iter_error f r

  Result.equal (=) (=) (Ok 1) (Ok 1)              = true
  Result.compare compare compare (Ok 5) (Ok 3)    = 1
  Result.compare compare compare (Ok 1) (Error _) = -1

30 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,13 @@ _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 — safe_div.ml baseline + Result.equal /
+  compare / iter_error (+3 tests, 592 total). safe_div divides only
+  if divisor non-zero, returns `Error "..."` otherwise. sum_safe folds
+  pairs with `Ok q -> acc+q | Error _ -> acc`.
+  `[(10,2);(20,4);(30,0);(50,5)]` → 5+5+0+10 = 20. Result additions:
+  equal/compare take separate eq/cmp for Ok and Error sides; Ok < Error
+  (-1) and Error > Ok (1). 30 baseline programs total.
 - 2026-05-09 Phase 6 — List.equal / List.compare (+5 tests, 589
   total). Both take an inner predicate / comparator and walk both
   lists in lockstep. equal short-circuits on first mismatch.