Summary

Add a reusable aggregation execution-stage base that buffers upstream events while
holding no resources, enriches each one inside a bounded resource scope, then
drains them. This lets any stage that enriches events via a point lookup amortize
per-event resource acquisition without holding acquisitions or locks across
getNext() calls. The primitive is enrichment-agnostic; it names no concrete
consumer.

Scope

• BatchedEnrichmentStage base: fill -> enrich -> drain loop; constructor-injected
  limits (max buffered events, max buffered input bytes, max enriched-output bytes
  with mid-input suspend); a memory tracker over both buffers that feeds operation
  memory accounting and drives the input byte cap; per-iteration interrupt checks;
  pass-through of control documents in arrival order; guaranteed scope close even
  when enrichment throws; a test failpoint.
• SingleDocumentLookupExecutor::releaseResources(): release attached state
  while cached plans survive; the PrimaryWithFallback combinator releases the primary
  before invoking the routed fallback and forwards the call to both children.
• ScopedBatchedLookup: RAII scope that reattaches the executor on construction
  and releases + detaches it on destruction; move-safe.

Out of scope

Wiring any concrete stage onto the base.

Testing

Unit tests only: combinator release ordering and forwarding; ScopedBatchedLookup
lifecycle including the exception path; stage fill/enrich/drain, the caps, output
suspend, pause/EOF propagation, control pass-through, memory-tracker accuracy, and
interrupt handling.