Support #[backtrace] fields in derived Error impls

[RAprogramm]

Summary

• teach the Error derive to generate backtrace/provide methods that expose #[backtrace] fields when the standard library feature is available
• validate #[backtrace] usage during input parsing and report duplicates or unsupported field types
• cover the new behaviour with unit tests and trybuild compile-fail cases

Testing

• cargo +nightly fmt --
• cargo clippy -- -D warnings
• cargo build --all-targets
• cargo test --all
• cargo doc --no-deps

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https://chatgpt.com/codex/tasks/task_e_68ccbb6ef4f0832baba7ab1ff6daa7f9