ocaml: phase 5.1 is_prime.ml baseline (count primes <= 100 = 25)

Trial division up to sqrt(n) with early-exit via bool ref:

  let is_prime n =
    if n < 2 then false
    else
      let p = ref true in
      let i = ref 2 in
      while !i * !i <= n && !p do
        if n mod !i = 0 then p := false;
        i := !i + 1
      done;
      !p

Outer count_primes loops 2..n calling is_prime, accumulating count.
Returns 25 — the canonical prime-counting function pi(100).

107 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -46,6 +46,7 @@
   "hanoi.ml": 1023,
   "hist.ml": 75,
   "int_sqrt.ml": 1027,
+  "is_prime.ml": 25,
   "fizz_classifier.ml": 540,
   "fizzbuzz.ml": 57,
   "flatten_tree.ml": 28,