Add coverage reporting and a non-regression guardrail

[parthdongre]

Summary

Make test coverage visible and prevent meaningful coverage regressions without chasing an arbitrary 100% target.

Why

The project already uses pytest-cov, but maintainers and contributors do not currently get a clear coverage signal in PRs or on the repository landing page.

Scope

• Measure the current baseline coverage for the maintained Python package.
• Publish coverage from CI using a service or GitHub-native artifact/reporting path.
• Add a small non-regression tolerance or agreed minimum threshold.
• Exclude generated/vendor code and clearly justify exclusions.
• Add a README badge only after the signal is stable.

Acceptance criteria

• PRs expose a coverage summary.
• The maintained src/framevitals package is measured consistently.
• CI fails when coverage drops below the agreed guardrail.
• Coverage configuration lives in version control.
• Contributor docs explain how to run the same coverage command locally.

Priority: P0 — testing / trust
Suggested labels: testing, quality