Normalize AppCode equality for custom codes

[RAprogramm]

Summary

• normalize AppCode to store Cow<'static, str> and compare by canonical string so literal and dynamic codes round-trip via serde
• switch pattern-based checks to equality in docs and tests where AppCode constants are used directly
• bump the crate version to 0.24.9 and document the change in the changelog

Testing

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

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https://chatgpt.com/codex/tasks/task_e_68d87e4243b4832ba67c972f2597ffec