Refine last4 masking to avoid temporary buffers

[RAprogramm]

Summary

• refactor the last4 masking helper to work directly with the Chars iterator and avoid a temporary Vec
• add a regression test that ensures multibyte strings retain their suffix under the last4 redaction policy

Testing

• cargo +nightly fmt --
• cargo +1.90.0 clippy --all-targets -- -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_68d658a328c4832b846ead153ef9415f