[Infra] Code coverage gate for the contract test suite

[Gbangbolaoluwagbemiga]

Goal

Track and enforce contract test coverage so the suite (#1) does not silently rot.

What needs to be done

• Add cargo llvm-cov to CI and generate an lcov/HTML report
• Upload the report as a CI artifact (and optionally to Codecov with a badge)
• Add a minimum threshold gate (start realistic, e.g. 70%, ratchet up over time)
• Document how to run coverage locally

Acceptance criteria

• Coverage report generated on every PR
• CI fails below the threshold
• Coverage badge in README (optional)

Relevant files

.github/workflows/node.yml, Cargo.toml

Depends on #1 and #25.