ocaml: phase 5.1 distinct_subseq.ml baseline ("rabbit" in "rabbbit" = 3)

Classic distinct-subsequences 2D DP:

  dp[i][j] = dp[i-1][j] + (s[i-1] = t[j-1] ? dp[i-1][j-1] : 0)
  dp[i][0] = 1   (empty t is a subseq of any prefix of s)

  count_subseq "rabbbit" "rabbit" = 3

The three witnesses correspond to which 'b' in "rabbbit" is
dropped (positions 2, 3, or 4 zero-indexed of the run of bs).

Complements subseq_check.ml (just tests presence); this one counts
distinct embeddings.

Tests 2D DP with Array.init n (fun _ -> Array.make m 0), base row
initialization, mixed string + array indexing.

175 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -34,6 +34,7 @@
   "csv.ml": 10,
   "egg_drop.ml": 8,
   "dijkstra.ml": 7,
+  "distinct_subseq.ml": 3,
   "exception_handle.ml": 4,
   "exception_user.ml": 26,
   "euler1.ml": 233168,