Add code coverage reporting for scripts/ with an enforced minimum threshold

[knytcomics-ui]

Problem

Once the unit/fuzz/integration test suites from other issues in this batch land, there's still no visibility into how much of scripts/ is actually exercised, and no guardrail preventing coverage from silently eroding as new scripts are added without matching tests.

Requirements

• Add c8 (Node's built-in-friendly coverage tool, pairs cleanly with node:test) as a dev dependency.
• Add npm run test:coverage producing a text summary and an lcov report.
• Set and enforce a minimum threshold (propose 90% given the small, testable surface area of this repo's scripts) that fails the command if not met.
• Add the coverage run to .github/workflows/ci.yml and upload the lcov report as a workflow artifact (or wire to a coverage-comment action) for PR visibility.

Acceptance Criteria

• npm run test:coverage fails if coverage drops below the configured threshold.
• Coverage report is visible from the CI run (artifact or PR comment).

Part of the Gryd-lock organization. This repo supplies the labelled testnet fixtures and stub scores consumed by grydlock-oracle-adapter's StubOracle and evaluated end-to-end by grydlock-research.

[jaynomyaro]

Hi! I’m a full-stack developer with a strong blockchain background. I’d love to handle this issue and can deliver a clean, reliable solution. Kindly assign it to me.