Package-level declarations

The Appendix B codec.

ProblemXml is the entry point: ProblemXml.encodeToString(problem) and ProblemXml.decodeFromString(text). Both are bounded by io.github.ilyankin.rfc9457.Problem.MAX_NESTING_DEPTH, the same constant the JSON codec uses, so neither format can accept a document the other refuses.

The namespace is urn:ietf:rfc:7807. That is not a typo and not a leftover: RFC 9457 kept RFC 7807's namespace deliberately. Emitting urn:ietf:rfc:9457 round-trips happily against itself and fails against everyone else.