ocaml: phase 5.1 levenshtein.ml baseline (no-memo edit distance, sum=11)

Recursive Levenshtein edit distance with no memoization (the test
strings are short enough for the exponential-without-memo version to
fit in <2 minutes on contended hosts). Sums distances for five short
pairs:

  ('abc','abx') + ('ab','ba') + ('abc','axyc') + ('','abcd') + ('ab','')
   = 1 + 2 + 2 + 4 + 2 = 11

Exercises:
  * curried four-arg recursion
  * s.[i] equality test (char comparison)
  * min nested twice for the three-way recurrence
  * mixed empty-string base cases

plans/ocaml-on-sx.md

@@ -407,6 +407,12 @@ _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 — levenshtein.ml baseline (recursive edit
+  distance, no memo). Sums distances for five short pairs:
+  ("abc","abx")=1 + ("ab","ba")=2 + ("abc","axyc")=2 +
+  ("","abcd")=4 + ("ab","")=2 = 11. Exercises curried four-arg
+  recursion + s.[i] equality test + min nested twice + mixed empty
+  string base cases.
 - 2026-05-09 Phase 5.1 — caesar.ml baseline (ROT13 with String.init +
   s.[i] + Char.code/chr). Side-quests:
   (1) top-level `let r = expr in body` is now treated as an expression