Summary

Introduce the LocalLookupEligibility interface and factory that decides, per event, whether a
change-stream lookup can be served locally. Ships the two trivial implementations needed for the
first optimized path; the real sharded routing module lands later (M4). No pipeline wiring yet –
consumed by the Express executor (later in this milestone).

Scope

• Define LocalLookupEligibility with a check() returning a decision that is either
  Local (carrying the shard/db version it was based on) or Unknown, plus an isLocal()
  helper.
• Define the LocalLookupEligibilityFactory interface (+ mock) so the decision strategy is
  injectable per stage.
• AlwaysLocalEligibility: always returns Local. Correct for replica sets, pre-images and
  $search.
• Always-decline implementation: always returns Unknown. The sharded placeholder, so sharded
  clusters route through the unchanged Aggregation fallback until M4.

Files

• src/mongo/db/exec/agg/single_doc_lookup/local_lookup_eligibility.h / .cpp
• src/mongo/db/exec/agg/single_doc_lookup/local_lookup_eligibility_factory_interface.h
• src/mongo/db/exec/agg/single_doc_lookup/local_lookup_eligibility_factory_impl.h / .cpp
• src/mongo/db/exec/agg/single_doc_lookup/local_lookup_eligibility_factory_mock.h
• src/mongo/db/exec/agg/single_doc_lookup/mock_local_lookup_eligibility.h
• src/mongo/db/exec/agg/single_doc_lookup/local_lookup_eligibility_test.cpp

Acceptance criteria

• Interface + factory defined; both implementations return the documented decision.
• Unit tests cover AlwaysLocal returning Local and the always-decline impl returning
  Unknown.