ocaml: phase 5.1 mortgage.ml baseline (monthly payment, 200k @ 5% / 30y = $1073)

Standard amortising-mortgage formula:

  payment = P * r * (1 + r)^n / ((1 + r)^n - 1)

where r = annual_rate / 12, n = years * 12.

  let payment principal annual_rate years =
    let r = annual_rate /. 12.0 in
    let n = years * 12 in
    let pow_r = ref 1.0 in
    for _ = 1 to n do pow_r := !pow_r *. (1.0 +. r) done;
    principal *. r *. !pow_r /. (!pow_r -. 1.0)

For 200,000 at 5% over 30 years: monthly payment ~= $1073.64,
int_of_float -> 1073.

Manual (1+r)^n via for-loop instead of Float.pow keeps the program
portable to any environment where pow is restricted.

Tests float arithmetic precedence, for-loop accumulation in a float
ref, int_of_float on the result.

70 baseline programs total — milestone.

lib/ocaml/baseline/expected.json

@@ -40,6 +40,7 @@
   "lambda_calc.ml": 7,
   "levenshtein.ml": 11,
   "memo_fib.ml": 75025,
+  "mortgage.ml": 1073,
   "merge_sort.ml": 44,
   "module_use.ml": 3,
   "newton_sqrt.ml": 1414,