ocaml: phase 5.1 fenwick_tree.ml baseline (BIT over 8 elements, fingerprint 228)

Fenwick / Binary Indexed Tree for prefix sums. The classic
`i & -i` low-bit trick needs negative-aware AND, but our `land`
evaluator (iter 127, bitwise via floor/mod arithmetic) only handles
non-negative operands. Workaround: a portable lowbit helper that
finds the largest power of 2 dividing i:

  let lowbit i =
    let r = ref 1 in
    while !r * 2 <= i && i mod (!r * 2) = 0 do
      r := !r * 2
    done;
    !r

After building from [1;3;5;7;9;11;13;15]:
  total       = prefix_sum 8 = 64
  update 1 by +100
  after       = prefix_sum 8 = 164
  total + after = 228

Tests recursive update / prefix_sum chains via helper-extracted
lowbit; documents a non-obvious limit of the bitwise-emulation
layer.

168 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -56,6 +56,7 @@
   "expr_eval.ml": 16,
   "expr_simp.ml": 22,
   "factorial.ml": 3628800,
+  "fenwick_tree.ml": 228,
   "fib_doubling.ml": 102334155,
   "fib_mod.ml": 391360,
   "fraction.ml": 7,