Summary

Implement the Express single-document lookup executor: a per-call _id-equality local point
lookup that serves the change-stream updateLookup against the user collection, declining
(kNotHandled) to the Aggregation fallback when the lookup is not provably local. No production
wiring here – wiring lands in a follow-up ticket.

Scope

• ExpressSingleDocumentLookupExecutor implementing SingleDocumentLookupExecutor
  (SERVER-128408).
• Eligibility-gated cheap decline: on an Unknown decision, return kNotHandled before any
  collection acquisition or plan build.
• Build a fresh executor per call via the existing public makeExpressExecutorForFindById /
  makeExpressExecutorForFindByClusteredId (from plan_executor_express.h), choosing the
  variant from the live collection layout.
• _id-only seek: strip any shard-key fields a sharded documentKey carries before building the
  find. Correct because _id is unique per shard and the read is local and versioned.
• Exception handling: StaleConfig / StaleDbVersion / ShardCannotRefreshDueToLocksHeld
  invalidate as appropriate and return kNotHandled; NamespaceNotFound /
  CollectionUUIDMismatch return kHandledNotFound; anything else propagates.
• Call assertLocalLookupReadAtOrAfter() (SERVER-128411) after acquisition for point-in-time
  correctness.
• Acquirer-agnostic: holds the polymorphic CollectionAcquirer base.

Files

• src/mongo/db/exec/agg/single_doc_lookup/express_single_document_lookup_executor.h / .cpp
• src/mongo/db/exec/agg/single_doc_lookup/single_document_lookup_executor_test.cpp (executor matrix)

Acceptance criteria

• All paths of the exception handling covered by unit tests, parameterised over the eligibility
  mock.
• A multi-field (sharded) documentKey is served by matching _id alone (kHandledFound), not
  declined for being sharded; a stale shard-key value in the documentKey is ignored.
• Cheap-decline verified: an Unknown decision returns kNotHandled with no acquisition.

Dependencies

Depends on SERVER-128408 (interface), SERVER-128411 (acquirer), SERVER-128412 (eligibility).