Align decimal --bwlimit rounding with upstream behaviour

[oferchen]

Summary

• track decimal suffix alignment in the bandwidth parser so MB/KB arguments round to decimal multiples instead of binary multiples
• adjust unit tests in the bandwidth crate and CLI to assert the corrected decimal results

Testing

• cargo test -p rsync-bandwidth
• cargo test -p rsync-cli -- --test-threads=1 --quiet
• cargo test -p rsync-core bandwidth_limit -- --nocapture
• cargo test -p rsync-daemon bwlimit -- --nocapture

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https://chatgpt.com/codex/tasks/task_e_68ffcf299fe08323b1297fa9bbe6e844