Summary

Add the SingleDocumentLookupExecutor abstraction that change-stream lookup strategies will implement, plus the PrimaryWithFallbackSingleDocumentLookupExecutor combinator that composes a primary strategy with a universal fallback. Interface and test mock only; no production wiring in this ticket.

Scope

• SingleDocumentLookupExecutor abstract base in src/mongo/db/exec/agg/single_doc_lookup/single_document_lookup_executor.h (header-only target, no SBE symbols).
• A three-state LookupResult with values kNotHandled, kHandledNotFound, kHandledFound.
• performLookup(expCtx, nss, collectionUUID, documentKey, afterClusterTime), where the last argument is boost::optional<Timestamp> (the event clusterTime, not a prebuilt readConcern).
• detachFromOperationContext() / reattachToOperationContext() lifecycle hooks.
• PrimaryWithFallbackSingleDocumentLookupExecutor: calls the primary; on kNotHandled it calls the fallback exactly once with identical arguments; forwards detach/reattach to both children.
• A test-only mock executor.

Acceptance criteria

• Unit tests for chain dispatch:
  • kHandledFound / kHandledNotFound return immediately; fallback not invoked.
  • kNotHandled invokes the fallback exactly once with the same arguments.
  • detach/reattach forwarded to both primary and fallback.
• No production callsite wired.