Extend the planner's streaming $group choice from SERVER-136171 to object _id keys whose members are all plain field paths, such as {{_id:

}}.

• Every member must be known not to be an array (nonArrayIdFields).
• The distinct source paths must be exactly the first fields of the access plan's base sort pattern, in any order and any directions.
• Nested objects, computed members, and equality-bound key fields still fall back to hash $group.