infra: Expand GitHub Actions workflow to include code coverage, more standard packages

[baylesj]

This PR modernizes the GitHub Actions CI workflow to add code coverage reporting, MSRV verification, documentation link checking, and build caching while streamlining cross-platform testing.

In this initial PR, code coverage is surfaced through codecov, which posts a comment posted back to the PR that caused the code coverage run to occur.

Key Changes

Code Coverage (using codecov)

• Measures source-based code coverage across the workspace using codecov, a pretty powerful and standard set of tools for doing code coverage analysis for public, open source repositories.

MSRV Verification

Added an msrv job verifying that the workspace compiles against Rust 1.85 (as specified in Cargo.toml).

Documentation Verification

Added a docs job running RUSTDOCFLAGS="-D warnings" cargo doc --workspace --all-features --no-deps to guard against broken intra-doc links and doc-build regressions.

Build Caching (Swatinem/rust-cache)

Added caching for ~/.cargo and build targets across matrix runners, significantly speeding up PR CI runs.

Standard Toolchain Management

Standardized toolchain and component installation with dtolnay/rust-toolchain and taiki-e/install-action.

Workflow Concurrency

Added concurrency.cancel-in-progress to automatically cancel superseded runs when pushing new commits to open PRs.

NOTE: Verified using a test PR in my repository fork: baylesj#2

[baylesj]

@pdeljanov verified using a PR in my Symphonia fork, but the bots are also running here so that should increase confidence. I think the most complex part of this YML configuration is the logic for generating the code coverage comment, but it is a good next step until we move to a more robust system (if desired).

[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 ☂️