Change license from BSD-2 to BSD-3 #125
Description
Standardize project licensing text on BSD-3-Clause and add an explicit non-endorsement clause to prevent misrepresentation of maintainer/project endorsement in derived products. There is no runtime or API impact for gem users; this is a legal/compliance clarification only. Redistributors should retain copyright and disclaimer text and must not imply endorsement by RubyTree maintainers/contributors.