Registry: <https://github.com/evolve75/RubyTree/pkgs/rubygems/rubytree>

Ensure children become true roots after remove_all! by clearing their parent
links. This matches the method's contract and prevents stale parent
references from corrupting later traversal or metrics.

Raise when renaming a child to an existing sibling name to prevent overwriting
entries in the children hash. This avoids orphaning nodes and keeps child
lookup consistent.

Fix set_child_at index errors and clean up parent/hash references
when replacing or clearing children. This preserves swap semantics and
avoids stale lookups.

Skip nil children during postorder and breadth-first traversals so binary
nodes with missing children do not break traversal. Add a regression test
covering nil child traversal paths.

Return a level-wise enumerator when no block is given and add a test to
assert the yielded level arrays match the tree structure.

Render '<Empty>' when a node has nil content and add tests for both
nil and non-nil content values.

Add a new 2.2.0 section covering recent behavior changes in tree
operations and traversal, plus link references for the new entries.

Bump the gem version to 2.2.0 and record the 2.2.0 changes in the
project history.