Add Arc-based error sources and lazy backtrace capture

[RAprogramm]

Summary

• store AppError sources in Arc<dyn Error> and gate backtrace capture through an env-aware lazy snapshot that respects the backtrace feature
• teach the derive macros, ResultExt, and ProblemJson conversion to reuse shared sources/backtraces without extra clones
• extend unit, integration, and trybuild coverage for chained sources, conditional backtraces, and update crate metadata (version + changelog)

Testing

• cargo +nightly fmt --
• cargo +1.90.0 clippy -- -D warnings
• cargo +1.90.0 build --all-targets
• cargo +1.90.0 test --all
• cargo +1.90.0 doc --no-deps
• cargo audit
• cargo deny check

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https://chatgpt.com/codex/tasks/task_e_68d2595314b8832b9e78b21f15bce929