Overview: This work introduces $_internalDocumentResultsAndMetadata, a new internal agg stage that serves as a transport container for source stages (e.g. $extensionSearch) that emit both a document-results stream and a metadata stream. It also lays the exec-layer foundation for 1:N DocumentSource-to-exec-stage translation.
 
Scope of Work
$_internalDocumentResultsAndMetadata stage skeleton.
  * New DocumentSource that wraps a source stage and fans its output into two consumer pipelines via DocumentSourceExchange.
  * Document stream flows into the top-level pipeline.
  * Metadata stream bound to a named pipeline variable (e.g. $$SEARCH_META) via $setVariableFromSubPipeline
  * On sharded queries, router serializes the stage with returnCursor: true: metadata returns as a secondary cursor, and the router injects $setVariableFromSubPipeline into the merging pipeline.
  * Includes explain, query stats, and auth support.
 
Metadata elision optimization
  * optimizeAt() removes the metadata consumer pipeline when no downstream stage references $$SEARCH_META, avoiding unnecessary exchange overhead.
 
1:N exec stage registry 
  * Extend exec::agg DocumentSourcetoStage registry to support 1:N mappings.
  * Allows a single DocumentSource to translate into multiple exec stages.
 
Translation function for $_internalDocumentResultsAndMetadata
  * Implement the translation function that produces N exec stages for $_internalDocumentResultsAndMetadata using the new 1:N registry.
 
Acceptance Criteria
  * $_internalDocumentResultsAndMetadata parses, serializes, and round-trips correctly in both standalone and sharded configurations.
  * $$SEARCH_META is correctly populated when the metadata field is present and referenced downstream.
  * Metadata consumer pipeline is elided when $$SEARCH_META is not referenced by any downstream stage.
  * exec::agg registry correctly handles 1:N mappings; existing 1:1 mappings are unaffected.
  * Translation of $_internalDocumentResultsAndMetadata produces the correct exec stage graph.
  * Auth, explain, and query stats paths all handle the new stage correctly.
  * All new unit tests pass; no regressions in existing test suites.