Optimize CI job structure

Structure your CI workflows to maximize performance and clarity. Each job should have a single, clear responsibility with these guidelines:

1. One toolchain per job: Keep jobs focused with a single Rust toolchain version per job. Multiple toolchains in one job create confusion about which version is being used for each command.

   # Don't do this:
   - name: Install Rust stable
     uses: dtolnay/rust-toolchain@stable
     with:
         toolchain: $
   - name: Install Rust nightly
     uses: dtolnay/rust-toolchain@stable
     with:
         toolchain: $
   

   Instead, create separate jobs for different toolchains or requirements.

2. Split long-running tasks: Divide lengthy operations into separate jobs that can run in parallel. When a job begins taking too much time (like Miri tests), split it to enable faster feedback cycles and better resource utilization.

3. Use efficient testing tools: Employ optimized testing tools like cargo-nextest to significantly reduce execution time. For example, replacing standard test runners with nextest can reduce CI runtime by 40% or more, as seen with Miri jobs (37 minutes → 21 minutes).

These practices will result in more maintainable CI configurations, faster feedback for developers, and clearer error identification when builds fail.