AdaptFlatFixture converts legacy golden fixture entities to the runtime
shape. Contract fixtures predate entity CRUD and store {id, field...}; Draft
collections store {id, fields:{field...}}. This is intentionally test-only
input adaptation, not a second runtime shape accepted by Records.
type Record struct {
ID string `json:"id"`
Fields map[string]any `json:"fields"`
}
Record is the only entity representation persisted in Draft collections.
Entity fields are deliberately nested so the stable entity ID cannot collide
with a Pack-defined field name.
Records reads one runtime entity collection. Invalid values are ignored so
complete validation can report the remaining configuration deterministically.