chore(release): bump version to 0.2.21

[suoak]

Summary

• bump the aionrs workspace and internal crates to 0.2.21
• update the release manifest and changelog
• establish the published dependency baseline for AionCore 0.1.84

Validation

• GitHub Actions format, lint, and test checks pass
• Cargo lockfile updated consistently
• Local release gate attempted; blocked before execution by the installed just version lacking which() support, so cloud validation is authoritative