ocaml: phase 5.1 lev_iter.ml baseline (sum of 5 edit distances = 16)

Iterative Levenshtein DP with rolling 1D arrays for O(min(m,n))
space. Distances:

  kitten    -> sitting    : 3
  saturday  -> sunday     : 3
  abc       -> abc        : 0
  ""        -> abcde      : 5
  intention -> execution  : 5
  ----------------------------
  total                   : 16

Complementary to the existing levenshtein.ml which uses the
exponential recursive form (only sums tiny strings); this one is
the practical iterative variant used for real ED.

Tests the recently-fixed <- with bare `if` rhs:

  curr.(j) <- (if m1 < c then m1 else c) + 1

153 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,15 @@ _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 — lev_iter.ml baseline (iterative
+  Levenshtein DP, sum of 5 distances = 16). Rolling-array DP
+  (O(min(m,n)) space). Distances: kitten→sitting=3, saturday→
+  sunday=3, abc→abc=0, ""→abcde=5, intention→execution=5;
+  3+3+0+5+5=16. Complementary to existing levenshtein.ml which
+  uses the recursive (exponential) definition; this one is the
+  practical iterative form used for real ED computations. Tests
+  the new `<- if ... then ... else ...` rhs path twice in the
+  same body. 153 baseline programs total.
 - 2026-05-10 Phase 5.1 — binary_heap.ml baseline (array-backed
   binary min-heap, push 9 random values then pop in sorted order
   → digits concat to 123456789). Standard heap mechanics: parent