Switch to c8 for code coverage (replace broken nyc/Istanbul)

Istanbul/nyc stopped working for us and we haven't had code coverage reporting since. Node.js now has native V8 coverage, and [`c8`](https://www.npmjs.com/package/c8) wraps it with an Istanbul-compatible reporter — simpler, faster, and no transpilation required.

## Ask

- Replace nyc/Istanbul with `c8` as the coverage runner for unit tests.
- Wire it into the CI unit-test run (once unit tests are in CI per [HarperFast/harper-pro#168](https://github.com/HarperFast/harper-pro/issues/168)).
- Set a reasonable coverage threshold that fails the build when coverage drops unexpectedly.

## Acceptance criteria

- `npm run test:unit` (with coverage flag) runs c8 and reports coverage.
- CI reports coverage per commit.
- Documented in CONTRIBUTING.

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Jira fields: **Feature Type:** Quality Assurance/Improvement · **Business Impact:** Operational efficiency *(both suggested)*

🤖 Filed by Claude on behalf of Kris.