ci: add per-contract test steps and coverage reporting

[devEunicee]

Closes #570

Summary

Updates .github/workflows/ci.yml to replace the single cargo test --all step with per-contract test runs and an LCOV coverage report.

Changes

• Added llvm-tools-preview component to the Rust toolchain install
• Replaced single cargo test --all with three named steps:
  • test — energy_token
  • test — audit_registry
  • test — community_governance
• Added taiki-e/install-action@cargo-llvm-cov to install cargo-llvm-cov
• Added cargo llvm-cov --all --lcov step that generates lcov.info
• Uploads the coverage report as a CI artifact (14-day retention)

Why per-contract steps?

When a single cargo test --all fails, it's not immediately obvious which contract is broken. Individual steps give a clear signal in the GitHub Actions UI.

Testing

CI will run on this PR. Coverage artifact will be visible in the Actions summary.