ocaml: phase 5.1 palindrome.ml baseline (two-pointer check, 4/6 inputs match)

Two-pointer palindrome check:

  let is_palindrome s =
    let n = String.length s in
    let rec check i j =
      if i >= j then true
      else if s.[i] <> s.[j] then false
      else check (i + 1) (j - 1)
    in
    check 0 (n - 1)

Tests on six strings:
  racecar = true
  hello   = false
  abba    = true
  ''      = true (vacuously, i >= j on entry)
  'a'     = true
  'ab'    = false

Sum = 4.

Uses s.[i] <> s.[j] (string-get + structural inequality), recursive
2-arg pointer advancement, and a multi-clause if/else if/else for
the three cases.

48 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 — palindrome.ml baseline (two-pointer
+  palindrome check, 4 of 6 inputs are palindromes). is_palindrome
+  walks from both ends meeting in the middle, returning false on
+  first mismatch. Tests on six strings (racecar ✓, hello ✗, abba ✓,
+  "" ✓, "a" ✓, "ab" ✗) sum to 4. Uses `s.[i] <> s.[j]` (string-get
+  + structural inequality) and recursive 2-arg pointer advancement.
+  48 baseline programs total.
 - 2026-05-09 Phase 5.1 — coin_change.ml baseline (min coin DP, 67¢
   with [1;5;10;25] → 6 coins). Bottom-up DP: `dp[i]` = min coins
   for amount i. For each amount 1..target, iterate coins and