fix(aggregation): preserve missing-reference behavior while keeping filter-only lookups

Body:
	•	refactor StageBuilder traversal/handlers for readability
	•	keep $lookup unfiltered for correct hydration; apply foreign predicates via local $filter
	•	emit explicit _missing_reference markers so deref raises DoesNotExist
	•	reduce duplication with shared helpers and structured dispatch

Author: arunsureshkumar