v0.6.4

Added

• Exposed an internal provide shim that mirrors thiserror's
  ThiserrorProvide, enabling derived errors to forward
  core::error::Request values to their sources.

Changed

• Allow #[backtrace] to be paired with #[source]/#[from] fields when the
  field type implements Error, while retaining diagnostics for incompatible
  non-source fields.
• Track whether backtrace detection is explicit or inferred so generated
  implementations avoid providing the same backtrace twice when delegating to
  sources.
• Update the generated provide methods to call thiserror_provide on source
  fields before exposing the stored backtrace, ensuring delegated traces reach
  callers.

Tests

• Added regression tests covering direct and optional sources annotated with
  #[backtrace], validating delegated backtrace propagation and None
  handling.