ocaml: phase 5.1 unique_paths_obs.ml baseline (4x4 grid w/ obstacles = 3)

Standard 2D unique-paths DP with obstacles gating each cell:

  dp[i][j] = if grid[i][j] = 1 then 0
             else dp[i-1][j] + dp[i][j-1]

Grid (1s are obstacles):
  . . . .
  . # . .
  . . . #
  # . . .

dp:
  1 1 1 1
  1 0 1 2
  1 1 2 0
  0 1 3 3

Returns dp[3][3] = 3.

Complements grid_paths.ml (no-obstacles version) — same DP shape
but obstacles zero out cells and reshape the path count.

199 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -196,6 +196,7 @@
   "triangle_div.ml": 120,
   "twosum.ml": 5,
   "union_find.ml": 4,
+  "unique_paths_obs.ml": 3,
   "unique_set.ml": 9,
   "validate.ml": 417,
   "word_count.ml": 3