feat: add Tarpaulin-based code coverage reporting

[Copilot]

Summary

Closes #136

Adds end-to-end code coverage reporting to the project using cargo-tarpaulin in CI and locally, with results uploaded to Codecov.

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Changes

.github/workflows/coverage.yml (new)

• New GitHub Actions workflow triggered on every push and pull request to main and dev.
• Installs cargo-tarpaulin, generates a Cobertura XML report covering the whole workspace (--workspace).
• Uploads the report to Codecov via codecov/codecov-action@v5 with token: ${{ secrets.CODECOV_TOKEN }}.
• Archives the raw coverage/ output as a build artefact on every run (actions/upload-artifact@v5).
• Uses Swatinem/rust-cache@v2 to cache Rust build artefacts and speed up repeated runs.

.tarpaulin.toml (new)

• Shared tarpaulin configuration so CI and the local helper script behave identically:
  • timeout = 300
  • ignore-panics = true
  • Excludes tests/* and benches/* from coverage.

scripts/coverage.sh (new)

• Convenience shell script for generating an HTML coverage report locally with a single command: ./scripts/coverage.sh.
• Cleans previous output, runs tarpaulin with --verbose, and auto-opens the report in the default browser (macOS / Linux).

README.md

• Added Codecov badge below the CI badge.
• Added a new 📈 Code Coverage section with:
  • Instructions for generating a report locally (script + direct tarpaulin command).
  • Coverage targets table (75 % minimum / 85 %+ desirable / 95 %+ for the marker-tracking critical path).

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Notes

• CODECOV_TOKEN is configured in the repo secrets and is active once merged to main; fail_ci_if_error: false prevents the upload step from blocking CI on this PR.
• Coverage is intentionally scoped to library code — tests/* and benches/* are excluded.

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

ℹ️ You can also turn on project coverage checks and project coverage reporting on Pull Request comment

Thanks for integrating Codecov - We've got you covered ☂️

[kalwalt]

ok ready to be merged.