Summary

Rebase the change-stream updateLookup stage onto the BatchedEnrichmentStage
primitive. Production runs a batch size of 1, so behaviour is unchanged – this
lands the batching machinery and its validation, not a behaviour change. Larger
production batches are turned on later, for collection-level streams, when the
caching (SBE) executor that makes batching pay is introduced.

Scope

• Reparent ChangeStreamUpdateLookupStage onto BatchedEnrichmentStage; enrich
  becomes the existing per-update-event post-image lookup (a minimal diff).
• Open/close a ScopedBatchedLookup per batch (a no-op on the per-call Express path).
• Add internal setParameters for the batch-size limits and pass them through the
  stage factory. The factory sets the production batch size to 1.

Validation

• Output byte-identical to the non-batched path; existing change_streams and
  passthrough suites green with the flag on and off.
• With the batch size forced above 1 via setParameter: a resume jstest that kills
  and resumes at forced batch-cut points with a non-empty buffer; a latency jstest
  confirming an idle stream still emits per event.

Depends on

SERVER-129168 (the primitive) and SERVER-128414 (the Express wiring).