[Florian-Cullmann]

Checklist

• The change is covered by existing issue Include compare information in the lateststorer plugin #4035.
• Corresponding unit tests are included.
• Changes are contained in a single commit.
• Update the documentation in another PR — documentation is currently included in this branch.
• npm test and npm run lint pass.

Description

Fixes #4035.

Latest test metadata now reports when the compare plugin is enabled and includes the baseline timestamp when a comparison completes. Both are also included in friendlyHTML for Grafana.

Metadata is written after comparisons finish and before uploads. When --compare.saveBaseline replaces the baseline file, the metadata preserves the date of the baseline used for comparison.

Validation:

• All 107 tests pass, including seven new regression tests.
• ESLint, Pug lint and the documentation build pass.
• Chrome smoke tests pass with and without an existing baseline, including baseline replacement.