Add ByteArea example and documentation

[somethingelseentirely]

Summary

• expand byte_area example to mutate multiple sections concurrently and showcase freezing the whole area or persisting it
• reference the example from README's advanced usage and examples list
• note the updated example in the changelog

Testing

• cargo run --example byte_area
• cargo test
• ./scripts/preflight.sh

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https://chatgpt.com/codex/tasks/task_e_688f663616f08322b6f05b5874797909