Add anyhow integration to gix-error

[Copilot]

Adds optional anyhow feature to gix-error enabling conversion of Exn<T> and Error types to anyhow::Error with proper source chain preservation.

Changes

• New IntoAnyhow trait - provides into_anyhow() method for converting to anyhow::Error

  • Implemented for both Exn<T> and Error
  • Uses trait pattern to avoid conflicts with anyhow's blanket From<E> implementation

• Tree-to-chain flattening - recursively follows first child of each frame to create linear source chain

  • Preserves all error information in owned ChainedFrameError wrappers
  • Enables proper "Caused by:" formatting in anyhow output

• Feature-gated module - src/anyhow.rs compiled only when anyhow feature enabled

Usage

use gix_error::{ResultExt, IntoAnyhow};

let result: Result<(), IoError> = Err(IoError("disk read failed"));
let result = result.or_raise(|| FileError("config.toml inaccessible"));
let result = result.or_raise(|| AppError("init failed"));

let anyhow_err = result.unwrap_err().into_anyhow();
// Prints:
// Error: init failed
// 
// Caused by:
//    0: config.toml inaccessible  
//    1: disk read failed

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[Byron]

Missed the mark.