Release Cedar 0.2.4

[oshtz]

Summary

• switch primary Windows and macOS release assets to stable filenames
• retain byte-identical legacy updater aliases for older Cedar clients
• migrate legacy Windows executable names to Cedar.exe after a healthy launch
• add packaged migration and artifact-contract verification

Verification

• cargo fmt --check
• cargo test --locked
• cargo clippy --locked --all-targets --all-features -- -D warnings
• scripts/package-windows.ps1
• PowerShell parser and actionlint v1.7.12