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.

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 — is_prime.ml baseline (count primes ≤ 100 =
+  25). Trial division up to √n with early-exit via bool ref. Loop
+  2..n calling is_prime, accumulate count. Returns 25 (the canonical
+  prime-counting function π(100)). Tests two cooperating functions
+  + while-with-bool-short-circuit + nested for. 107 baseline
+  programs total.
 - 2026-05-09 Phase 5.1 — catalan.ml baseline (Catalan number C(5)
   via DP recurrence = 42). DP recurrence `C(n) = sum_{j=0}^{n-1}
   C(j) * C(n-1-j)`. C(5) = 42 — also the count of distinct binary