Use durable atomic replace for Unix self-updates

[ryanfowler]

Summary

• Route self-update unpacking by archive suffix instead of compile-time OS branches.
• Change non-Windows replacement to copy the unpacked executable into the target directory and replace it with fileutil::atomic_replace_file, preserving the Unix parent-directory sync.
• Add regression tests for suffix-based archive dispatch, unsupported archive formats, and non-Windows replacement behavior.

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo test --all-features --test integration -- --test-threads=1