Synchronize Cargo version during releases

[lukewilliamboswell]

Summary

• validate the dispatched release version once, then synchronize Cargo.toml and Cargo.lock before every host build
• verify the synchronized version through cargo metadata, including prerelease versions and a clear mismatch diagnostic
• create a release-source commit with the Cargo version update and target the Git tag at that exact commit
• base the existing example follow-up update on the tagged commit so one follow-up PR contains both Cargo and example changes
• replace the 0.0.1 placeholder with the current 0.21.0-rc4 platform version

Testing

• python3 -m unittest scripts/test_check_release_version.py
• python3 scripts/check_release_version.py 0.21.0-rc4 --update
• mismatch diagnostic exercised with 0.21.0
• cargo check --locked
• ./scripts/test.py --operation validate

Closes #447