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

lib/ocaml/baseline/unique_paths_obs.ml

@@ -0,0 +1,28 @@
+let h = 4
+let w = 4
+let grid = [|
+  [| 0; 0; 0; 0 |];
+  [| 0; 1; 0; 0 |];
+  [| 0; 0; 0; 1 |];
+  [| 1; 0; 0; 0 |]
+|]
+
+let paths_with_obs () =
+  let dp = Array.init h (fun _ -> Array.make w 0) in
+  if grid.(0).(0) = 0 then dp.(0).(0) <- 1;
+  for j = 1 to w - 1 do
+    if grid.(0).(j) = 0 then dp.(0).(j) <- dp.(0).(j - 1)
+  done;
+  for i = 1 to h - 1 do
+    if grid.(i).(0) = 0 then dp.(i).(0) <- dp.(i - 1).(0)
+  done;
+  for i = 1 to h - 1 do
+    for j = 1 to w - 1 do
+      if grid.(i).(j) = 0 then
+        dp.(i).(j) <- dp.(i - 1).(j) + dp.(i).(j - 1)
+    done
+  done;
+  dp.(h - 1).(w - 1)
+;;
+
+paths_with_obs ()