ExUnit formatter which writes the Atlas JSON and HTML reports.

Register it alongside the standard CLI formatter in test/test_helper.exs:

ExUnit.start(
  formatters: [
    ExUnit.CLIFormatter,
    ExUnitAtlas.Formatter
  ]
)

ExUnit owns this module's lifecycle; do not start it directly. Reports are written to ex_unit_atlas_report/report.json and ex_unit_atlas_report/index.html. The CLI formatter remains responsible for terminal output and the test process exit status.