ocaml: phase 5.1 pi_leibniz.ml baseline + int_of_float fix (1000 terms x 100 = 314)

pi_leibniz.ml: Leibniz formula for pi.

  pi/4 = 1 - 1/3 + 1/5 - 1/7 + ...
  pi  ~= 4 * sum_{k=0}^{n-1} (-1)^k / (2k+1)

For n=1000, pi ~= 3.140593. Multiply by 100 and int_of_float -> 314.

Side-quest: int_of_float was wrongly defined as identity in
iteration 94. Fixed to:

  let int_of_float f =
    if f < 0.0 then _float_ceil f else _float_floor f

(truncate toward zero, mirroring real OCaml's int_of_float). The
identity definition was a stub from when integer/float dispatch was
not yet split — now they're separate, the stub is wrong.

Float.to_int still uses floor since OCaml's docs say the result is
unspecified for nan / out-of-range; close enough for our scope.

32 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -23,6 +23,7 @@
   "module_use.ml": 3,
   "mutable_record.ml": 10,
   "option_match.ml": 5,
+  "pi_leibniz.ml": 314,
   "poly_stack.ml": 5,
   "queens.ml": 2,
   "quicksort.ml": 44,