Summary

First consumption of featureFlagChangeStreamOptimizedUpdateLookup (SERVER-117887). Wire the
Express executor live for every replica-set / non-sharded change stream so local updateLookups
bypass the heavyweight routed lookupSingleDocument. This is the first optimized path and the
first new behaviour.

Scope

• Gate the executor factory in change_stream_add_post_image_stage_fn.cpp:
  • Flag off -> Aggregation-only chain (M1 behaviour, byte-for-byte master).
  • Flag on, replica set / non-sharded -> AlwaysLocal + Express, with the Aggregation fallback.
  • Flag on, sharded -> always-decline eligibility, so the lookup routes through the unchanged
    Aggregation fallback.
• Enable Express uniformly across collection / db / cluster streams. Per-stream-type refinement
  (collection-level -> SBE) is deferred to M3.

Files

• src/mongo/db/exec/agg/change_stream_add_post_image_stage_fn.cpp

Acceptance criteria

• With the flag off, output is byte-for-byte master.
• With the flag on, replica-set change_streams and change_streams_secondary_reads suites
  are green and the Express path is exercised; fullDocument is byte-identical to master.
• Sharded deployments provably fall back (no behaviour change).

Dependencies

Depends on SERVER-117887 , SERVER-128410 (Aggregation-only rewire), SERVER-128413 (Express
executor).