Add support for code coverage

[saulshanabrook]

This PR adds support for running the tests locally to produce code coverage and in CI uploading to Codecov.

This should allow us to see for each PR how it affects the overall code coverage and what added lines are covered by tests/examples. I have been finding myself wanting this, as I develop new features to make sure I covered all the cases with examples.

I don't suggest we require 100% coverage for new lines added. However, even just seeing the lines can help you understand if your tests are covering what you think they are and possibly encourage covering more lines.

It can also help see what parts of our code base are not used in any examples.

[Copilot]

Pull request overview

This PR adds code coverage support to the egglog project by integrating cargo-llvm-cov for local coverage generation and Codecov for CI-based coverage reporting and tracking.

Changes:

• Added Codecov badge to README and comprehensive documentation for generating coverage reports locally
• Created new coverage and doctest Makefile targets to support coverage generation
• Integrated coverage reporting into CI workflow with automatic uploads to Codecov

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
README.md	Added Codecov badge, new Code Coverage section with local setup instructions, reorganized Parallelism section, and improved consistency by capitalizing "CodSpeed"
Makefile	Added coverage target using cargo-llvm-cov, extracted doctest as separate target, updated .PHONY declarations
.gitignore	Added lcov.info to ignore list for generated coverage reports
.github/workflows/build.yml	Updated test job to install cargo-llvm-cov, run coverage generation, and upload results to Codecov

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[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing saulshanabrook:code-coverage (14640ee) with main (1424a89)}

Summary

✅ 22 untouched benchmarks
⏩ 190 skipped benchmarks¹

Footnotes

1. 190 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[codecov-commenter]

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.

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