e.g. "memberJoined", "memberRemoved", "roleChangedToAdmin".
DID of the actor who performed the change (empty if unattributed).
DID of the member the change is about (empty if not yet known).
base64 encrypted_member_id of the target, when the action names one.
Ready-to-log English one-liner (uses DIDs, not display names).
A membership/metadata change derived from a group's change log (docs/03 §3.6) — e.g. someone joined, left, was added/removed, or a role changed. app-core has already persisted the matching system row; bots can act on the structured fields (e.g. adminbot noting it was added to a group) or log summary directly.