Shrink AppError footprint

[RAprogramm]

Summary

• move optional source/backtrace storage into the boxed ErrorInner so the default AppResult Err variant stays small
• add a regression test that enforces the AppError size budget
• bump the crate to 0.21.1 and refresh the changelog and README snippets

Testing

• cargo +nightly-2025-02-15 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 +1.90.0 deny check
• cargo +1.90.0 audit

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https://chatgpt.com/codex/tasks/task_e_68d3bce6829c832bbf7e5a066f182bdb