The record this composition was built for, as CompositeKey.ToURLSegment(), which
CompositeKey.FromURLSegment reads back. Null for a record not saved yet.
AdditionalContext is app-global and replaced wholesale by whichever surface
published last, so a consumer needs to be able to tell which record a snapshot
describes rather than assuming it matches the conversation.
The side rail this form shows, in order, with the panels filed under each item.