ocaml: phase 5.1 binary_heap.ml baseline (min-heap sort 9 vals -> 123456789)

Array-backed binary min-heap with explicit size tracking via ref:

  let push a size x =
    a.(!size) <- x; size := !size + 1; sift_up a (!size - 1)

  let pop a size =
    let m = a.(0) in
    size := !size - 1;
    a.(0) <- a.(!size);
    sift_down a !size 0;
    m

Push [9;4;7;1;8;3;5;2;6], pop nine times -> 1,2,3,4,5,6,7,8,9.
Fold-as-decimal: ((((((((1*10+2)*10+3)*10+4)*10+5)*10+6)*10+7)*10+8)*10+9 = 123456789.

Tests recursive sift_up + sift_down, in-place array swap,
parent/lchild/rchild index arithmetic, combined push/pop session
with refs.

152 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -11,6 +11,7 @@
   "bf_full.ml": 6,
   "bisect.ml": 141,
   "bigint_add.ml": 28,
+  "binary_heap.ml": 123456789,
   "bits.ml": 21,
   "balance.ml": 3,
   "base_n.ml": 17,