ocaml: phase 5.1 grid_paths.ml baseline (count paths in 4x6 grid = 210)

DP filling a flattened 2D array:

  dp.(0, 0)         = 1
  dp.(i, j)         = dp.(i-1, j) + dp.(i, j-1)
  index             = i * (n+1) + j

For a 4x6 grid (5x7 dp matrix), the count is C(10, 4) = 210.

Tests Array as 2D via row-major flatten + nested for + multi-step
conditional access (above/left guarded by 'if i > 0' / 'if j > 0').

87 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -31,6 +31,7 @@
   "frequency.ml": 5,
   "gcd_lcm.ml": 60,
   "grep_count.ml": 3,
+  "grid_paths.ml": 210,
   "group_consec.ml": 53,
   "hailstone.ml": 111,
   "hanoi.ml": 1023,