Release v0.5.1

[sethrj]

Pre-merge checklist

• Ensure all CI jobs on the branch to be released (develop or backports/vX.Y) pass. https://github.com/celeritas-project/celeritas/actions/runs/12776904916
• Run performance regression tests on Perlmutter, Frontier, and an additional machine with debug assertions enabled (e.g., Wildstyle). celeritas-project/regression@ca92064
• Update documentation with release notes from all pull requests newly included in the release.
• Ensure PDF documentation builds without error.

Post-merge checklist

• If releasing a backported version branch, cherry-pick this documentation commit into the backport branch.
• Use the GitHub interface to create a new release with the documentation update that was just added.

Post-release checklist

• Save the tar.gz and attach to the release, because the hash changes if the git "describe" function returns a different result for the release tag's hash (e.g., if a collaborative branch on the main repository points to that commit).
• Pull locally (make sure to use the --tags option) and build PDF user documentation for the release. Ensure breathe is activated (so the API is listed) and that the version is embedded correctly.
• Update the Spack recipe for Celeritas with the new version and sha256 value (either manually or using spack checksum) and submit a pull request to the Spack project.
• Mark the GitHub release milestone as completed.

[github-actions]

Test summary

 4 213 files   6 515 suites   4m 51s ⏱️
 1 630 tests  1 601 ✅ 29 💤 0 ❌
21 545 runs  21 465 ✅ 80 💤 0 ❌

Results for commit efd0e96.

♻️ This comment has been updated with latest results.

[sethrj]

Waiting on #1586 and #1578.

[sethrj]

Looks like ORANGE is slightly slower on CPU on both platforms? But there shouldn't be any real changes...