fix: skip self-update when binary already matches latest asset

[thomasjiangcy]

Summary

• avoid unnecessary executable replacement in pkgrep self update
• after downloading and extracting the latest release asset, compare it against the current installed binary
• if bytes match, exit as no-op with clear status message
• add checksum parser test coverage for *filename checksum format tokens
• add changelog note under Unreleased

Why

Current binary version strings can lag release tags in some workflows. This follow-up prevents repeated self-update rewrites when the installed binary already matches the latest release artifact.

Validation

• cargo test --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• ./.dev/check_no_mocks.sh