Children are listed in subtree order (escaped segment + separator) so the
skip-scan stays O(children) even when sibling keys extend each other with
characters below the separator. Model-based property test vs a reference Map.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Qxbkp4tA7G5SCWjzWuxmE