ocaml: phase 5.1 manacher.ml baseline (longest palindrome "babadaba" = 7)

Manacher's algorithm: insert # separators (length 2n+1) to unify
odd/even cases, then maintain palindrome radii p[] alongside a
running (center, right) pair to skip work via mirror reflection.
Linear time.

  manacher "babadaba" = 7   (* witness: "abadaba", positions 1..7 *)

Note: requires parenthesizing the if-expression on the rhs of <-:

  p.(i) <- (if pm < v then pm else v)

Real OCaml parses bare `if` at <-rhs since the rhs is at expr
level; our parser places <-rhs at binop level which doesn't include
`if` / `match` / `let`. Workaround until we relax the binop
RHS grammar.

149 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -87,6 +87,7 @@
   "lambda_calc.ml": 7,
   "lcs.ml": 4,
   "majority_vote.ml": 4,
+  "manacher.ml": 7,
   "levenshtein.ml": 11,
   "memo_fib.ml": 75025,
   "mortgage.ml": 1073,