Summary

Implement the universal fallback executor: the existing routed lookup mechanism (a routed match on the documentKey via MongoProcessInterface::lookupSingleDocument) wrapped behind the SingleDocumentLookupExecutor interface. This is the correctness floor every fast path can decline to.

Scope

• AggregationSingleDocumentLookupExecutor in src/mongo/db/exec/agg/single_doc_lookup/aggregation_single_document_lookup_executor.h / .cpp.
• Yields all locks before routing (the single lock-free routing site).
• Builds a majority readConcern with afterClusterTime only when routing, and none when afterClusterTime is absent.
• Always returns kHandledFound or kHandledNotFound, never kNotHandled, so a PrimaryWithFallback chain always resolves.

Acceptance criteria

• Unit tests for found / not-found outcomes and readConcern construction (present iff afterClusterTime is present).
• Depends on the SingleDocumentLookupExecutor interface ticket (SERVER-128408).