release: Rivet 0.7.0

[turinglambdaai]

Summary

• cut the completed 0.7 production-graduation work as Rivet 0.7.0
• keep the Racket package version valid as 0.7 while publishing three-component release version 0.7.0
• move the accumulated unreleased notes under the 0.7.0 changelog heading
• update the English and Chinese version badges

Validation

• isolated catalog install of Keel 0.3.1 and linked Rivet
• raco test tests/ — 8,942 tests passed
• raco rivet help
• raco setup --check-pkg-deps rivet, including documentation build
• git diff --check