ocaml: phase 5.1 count_paths_dag.ml baseline (source-to-sink paths = 3)

Count source-to-sink paths in a DAG via Kahn's topological sort
plus accumulation:

  paths[source] = 1
  for u in topological order:
    for v in adj[u]: paths[v] += paths[u]

Same 6-node DAG as topo_sort.ml:
  0 -> {1, 2}    1 -> {3}    2 -> {3, 4}    3 -> {5}    4 -> {5}

The three witnesses 0 -> 5:
  0 -> 1 -> 3 -> 5
  0 -> 2 -> 3 -> 5
  0 -> 2 -> 4 -> 5

Tests Queue-driven Kahn order + List.rev to recover topological
order, module-level mutable arrays (in_deg, paths), accumulation
in topological traversal.

191 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -31,6 +31,7 @@
   "coin_change.ml": 6,
   "coin_min.ml": 6,
   "count_change.ml": 406,
+  "count_paths_dag.ml": 3,
   "count_inversions.ml": 12,
   "count_palindromes.ml": 9,
   "count_subarrays_k.ml": 7,