v0.5.15

Added

• Parse #[error("...")] attribute arguments into structured FormatArg
  entries, tracking named bindings and positional indices for future
  format_args! integration.
• Recognise #[error(fmt = <path>)] handlers, capturing the formatter path and
  associated arguments while guarding against duplicate fmt specifications.

Fixed

• Produce dedicated diagnostics when unsupported combinations are used, such as
  providing format arguments alongside #[error(transparent)].

Tests

• Extend the trybuild suite with regression cases covering duplicate fmt
  handlers and transparent attributes that erroneously include arguments.