Cargo monorepo 🙈

Make it a monorepo. Then it's possible to auto check cargo build with github checks.

The issue is to restructure the repository into a Cargo monorepo to enable automated GitHub checks. Start by examining the current project structure and Cargo.toml files. Research Cargo workspace configuration and how to set up GitHub Actions for Rust builds. Determine what 'done' looks like: a working workspace with all crates building and passing checks in CI.

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