ci: publish repository coverage via codecov

[Ramlaoui]

Summary

• upload Python and Rust coverage reports to Codecov from CI
• show the repository coverage badge at the top of README.md
• remove the custom PR coverage summary script and README coverage section

Testing

• validated .github/workflows/ci.yml parses
• not run full project test suites locally

Notes

• the README badge tracks the main branch coverage in Codecov
• PR coverage updates come from Codecov uploads in the CI workflow

[github-actions]

Coverage Report

Scope	Covered Lines	Total Lines	Line Coverage
Python	221	623	35.47%
Rust	1286	5200	24.73%

Artifacts:

• Python XML/HTML coverage: atompack-py/coverage/
• Rust LCOV coverage: coverage/rust.lcov

[Ramlaoui]

This is blocked on Codecov onboarding, not on the coverage generation code itself.

Current status:

• the workflow can generate both Python and Rust coverage reports
• the repository coverage badge / PR reporting cannot be completed until LeMaterial/atompack is enabled in Codecov
• that requires org-level GitHub App access or someone who already has Codecov org access to enable the repo and provide the upload token

Concretely, we cannot finish the Codecov path unless one of these happens:

1. a LeMaterial org admin grants the Codecov GitHub App access to atompack
2. the repo is already enabled in Codecov and someone provides the CODECOV_TOKEN
3. we abandon Codecov and switch to a GitHub-only badge flow instead

Marking this PR as draft until that dependency is resolved.