v0.12.0

Added

• Introduced typed Metadata storage with Field/FieldValue builders and helper functions in field::*.
• Captured error sources and backtraces inside the new app_error::Error container, exposing MessageEditPolicy to control redaction.

Changed

• Replaced the legacy AppError struct with the richer Error model carrying AppCode, metadata, retry/auth hints and transport policy.
• Updated response mapping and constructors to preserve machine-readable codes without extra allocations.

Documentation

• Refreshed crate docs, README (EN/RU) and examples to highlight metadata helpers and the new error contract.

Tests

• Added regression coverage ensuring codes, metadata and sources survive conversions without unnecessary cloning.