v0.22.0

Added

• Introduced an explicit std feature (enabled by default) and made the core
  crate compile in no_std + alloc environments, including metadata builders
  and error helpers.

Changed

• Reworked AppError internals to rely on core/alloc primitives and
  core::error::Error, providing std::error::Error only when the std
  feature is active.
• Replaced thiserror derives on AppErrorKind with manual Display/error
  implementations so the taxonomy remains available without the standard
  library.