Add print_tree IO support docs/tests (#69)

Capture print_tree IO support in History and add a print_tree_to_s test
coverage entry alongside the IO-based print_tree test.

Author: evolve75

History.md

@@ -16,6 +16,9 @@
 * Add `cmp` for traversal/relationship-based comparison without changing the
   name-based `<=>` semantics.
 
+* Allow `print_tree` to write to a custom IO and add `print_tree_to_s` for
+  string output.
+
 ### 2.2.1pre / 2026-02-07
 
 * Simplified development dependency constraints while maintaining Ruby 2.7+

test/test_tree_traversal.rb

@@ -243,8 +243,14 @@ def test_collect
     def test_print_tree
       setup_test_tree
       buffer = StringIO.new
-      @root.print_tree(0, nil, ->(node, prefix) { buffer.puts "#{prefix} #{node.name}" })
+      @root.print_tree(0, nil, nil, io: buffer)
       assert(buffer.string.include?('ROOT'))
     end
+
+    def test_print_tree_to_s
+      setup_test_tree
+      output = @root.print_tree_to_s
+      assert(output.include?('ROOT'))
+    end
   end
 end