chore(ci): update GitHub Actions versions

[DCjanus]

Why

• Recent workflow runs warn that several actions are still running on Node.js 20, which GitHub will force-upgrade to Node.js 24 on June 2, 2026 and remove from runners on September 16, 2026.
• The README can also expose the crate dependency status next to the existing status badges.

What

• Update actions/checkout from v3 to v6 across workflows.
• Update the docs deployment action from JamesIves/github-pages-deploy-action@releases/v3 to v4 and switch to the current input names.
• Update codecov/codecov-action from v3 to v6.
• Add the deps.rs dependency status badge to the README.

Validation

• git diff --check
• ruby -e 'require "yaml"; ARGV.each { |f| YAML.load_file(f); puts "OK #{f}" }' .github/workflows/*.yml
• actionlint

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.98%. Comparing base (8598600) to head (0eb2cc6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #189   +/-   ##
=======================================
  Coverage   80.98%   80.98%           
=======================================
  Files          23       23           
  Lines        1530     1530           
=======================================
  Hits         1239     1239           
  Misses        291      291           

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[Copilot]

Pull request overview

This PR modernizes the repository’s GitHub Actions workflows to newer major versions (addressing upcoming runner/runtime deprecations) and improves project visibility by adding a dependency-status badge to the README.

Changes:

• Bumped actions/checkout from v3 to v6 across all workflows.
• Updated docs deployment from JamesIves/github-pages-deploy-action@releases/v3 to @v4, including updated input names and added contents: write permissions.
• Updated Codecov upload action from codecov/codecov-action@v3 to @v6, and added a deps.rs badge to the README.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
README.md	Adds deps.rs dependency status badge alongside existing project badges.
.github/workflows/docs.yml	Updates checkout + docs deploy action to newer versions; aligns inputs and sets required permissions for branch deployment.
.github/workflows/coverage.yml	Updates checkout and Codecov actions to newer major versions.
.github/workflows/clippy.yml	Updates checkout action to v6 for clippy/fmt jobs.
.github/workflows/build.yml	Updates checkout action to v6 for the build/test matrix.

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