chore(cargo): bump version to 0.4.2

[rocketman-code]

Summary

• Bump version to 0.4.2
• Add changelog entry for v0.4.2

What's in 0.4.2

• Animated demo GIF in README (Add animated demo GIF to README #184)
• JSON field naming consistency: DiffPackageEntry.size -> total_size_bytes (fix(report): consistent field names across package report types #185)
• Eliminated all process::exit from main.rs, replaced with ExitCode returns (fix(cli): replace process::exit with ExitCode returns #186)
• Centralized JSON/terminal output dispatch via report::emit(), added --json to diff subcommand (fix(cli): respect --json flag in all diff output paths #187)
• Closed bug(repl): help text missing --json flag documentation #152 as already fixed