Configure full Rust CI workflow

[bmuddha]

Outcome

Pull requests run the standard Rust checks in CI.

Scope

CI may use parallel jobs and should use a high-performance caching layer such as sccache. It must run:

1. cargo fmt --check
2. cargo clippy --all-features --all-targets -- -D warnings
3. cargo nextest run --nff

Acceptance criteria

• Every pull request that includes the workflow triggers the CI run.
• All three Rust checks report their result independently.