ocaml: phase 5.1 union_find.ml baseline (10 nodes, 6 unions, 4 components)

Disjoint-set union with path compression:

  let make_uf n = Array.init n (fun i -> i)
  let rec find p x =
    if p.(x) = x then x
    else begin let r = find p p.(x) in p.(x) <- r; r end
  let union p x y =
    let rx = find p x in let ry = find p y in
    if rx <> ry then p.(rx) <- ry

After unioning (0,1), (2,3), (4,5), (6,7), (0,2), (4,6):
  {0,1,2,3} {4,5,6,7} {8} {9}  --> 4 components.

Tests Array.init with closure, recursive find, in-place .(i)<-r.

139 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,13 @@ _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.
+- 2026-05-10 Phase 5.1 — union_find.ml baseline (disjoint-set union
+  on n=10 with 6 unions → 4 components). Path-compressing find:
+  recursively walks parent links, splices subtree onto root in place.
+  After unioning {0-1, 2-3, 0-2} → {0,1,2,3}; {4-5, 6-7, 4-6} →
+  {4,5,6,7}; 8 and 9 remain singletons → 4 components. Tests
+  Array.init, in-place .(i)<-r mutation, recursive find with
+  path compression. 139 baseline programs total.
 - 2026-05-10 Phase 5.1 — quickselect.ml baseline (Hoare quickselect
   median of [7;2;9;1;5;6;3;8;4] = 5). Lomuto partition scheme:
   recursively partitions on the last element as pivot, narrows the