ocaml: phase 5.1 island_count.ml baseline (6x7 grid, 5 components)

Count 4-connected components of 1-cells via DFS flood from every
unvisited 1-cell.

Grid (1s shown as #):
  # # . . . # #
  # . . . # # .
  . . # # . . .
  . # # # . # #
  # . . . . # .
  # # . # # # .

Components:
  {(0,0),(0,1),(1,0)}
  {(0,5),(0,6),(1,4),(1,5)}
  {(2,2),(2,3),(3,1),(3,2),(3,3)}
  {(3,5),(3,6),(4,5),(5,3),(5,4),(5,5)}
  {(4,0),(5,0),(5,1)}
                              -> 5 islands

Complementary to flood_fill.ml (largest component); this counts
total components.

Tests recursive function returning () at early-exit branches,
ordered double-for entry pass triggering one fill per island root.

180 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,18 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-11 Phase 5.1 — island_count.ml baseline (count 4-connected
+  components of 1-cells in a 6×7 grid = 5). DFS flood from every
+  unvisited 1-cell. Counted islands:
+    {(0,0),(0,1),(1,0)}
+    {(0,5),(0,6),(1,4),(1,5)}
+    {(2,2),(2,3),(3,1),(3,2),(3,3)}
+    {(3,5),(3,6),(4,5),(5,3),(5,4),(5,5)}
+    {(4,0),(5,0),(5,1)}
+  Complementary to flood_fill.ml which measures largest component;
+  this counts them. Tests recursive returning `()` (unit) at
+  early-exit branches, ordered double-for entry pass that triggers
+  one fill per island root. 180 baseline programs total.
 - 2026-05-11 Phase 5.1 — dp_word_break.ml baseline (word-break DP
   over 5 strings with 9-word dictionary; 4 strings segmentable).
   dp[i] = ∃ word w of length wl ≤ i with prefix s[i−wl..i]=w and