feat(ci): add binding coverage uploads

[chaliy]

Summary

• add Python-driven Rust coverage collection through the binding test suite
• add Node-driven Rust coverage collection through the NAPI build and Node tests
• upload all Rust coverage artifacts together to Codecov and document the expanded coverage model in specs/004-testing.md

Validation

• YAML parse check for .github/workflows/coverage.yml
• git diff --check
• end-to-end workflow validation will come from GitHub Actions on this PR

[codecov]

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