Issue #3609156: Add a kessai_worldline README documenting webhook settlement and setup

Frank Mablyrequested to merge
issue/kessai-3609156:3609156-add-a-kessaiworldline into 1.x

Adds kessai_worldline/README.md: the two settlement channels (browser return and HMAC-verified server-to-server webhook), the abandoned-browser guarantee and how it resumes an orchestra workflow step server-side, keeping secrets in settings.php, and a checklist for enabling the webhook. Adds Worldline, HMAC and finalizer to the cspell dictionary.

Closes #3609156.

Merge request reports