v0.13.0

Added

• Introduced #[derive(Masterror)] and the #[masterror(...)] attribute to
  convert domain errors directly into [masterror::Error] while capturing
  metadata, message redaction policy and optional transport mappings.
• Added transport mapping descriptors in mapping::{HttpMapping, GrpcMapping, ProblemMapping} generated by the new derive for HTTP/gRPC/problem-json
  integrations.

Changed

• Re-exported the Masterror derive from the crate root alongside the existing
  Error derive.

Documentation

• Expanded crate docs and both READMEs with Masterror examples, telemetry
  guidance and redaction policy notes.

Tests

• Added integration tests and trybuild coverage exercising the
  #[masterror(...)] attribute and generated mapping tables.