fix(update): avoid flaky timestamp assertions

[ryanfowler]

Summary

• compare executable inode identity after self-update instead of modification timestamps
• keep timestamp assertions for no-op and dry-run update cases
• avoid macOS CI flakes when replacement files receive identical timestamps

Validation

• cargo fmt --check
• cargo test --locked --all-features --test update
• cargo test --locked --all-features --lib --bins

Caveat

• Full clippy currently fails on an unrelated existing dead-code warning in src/dns/wire.rs.