v0.12.1

Added

• Introduced the Context builder for enriching error conversions with
  metadata, caller tracking, and redaction hints via ResultExt::ctx.
• Implemented the ResultExt trait to wrap fallible operations into
  masterror::Error without extra allocations while merging context fields.

Documentation

• Added rustdoc examples showcasing Context chaining and the new
  ResultExt helper.

Tests

• Added unit coverage for ResultExt::ctx, ensuring happy-path results pass
  through and error branches preserve metadata and sources.