Mapping Services Lazy-Loading Integration

[JeremyCaney]

The mapping services (both forward and reverse) fan out as concurrent tasks assuming loads complete synchronously, which was true before lazy loading, but is now false—so awaits expose concurrency races, nondeterministic ordering, and cases where checks silently force full eager loads, which defeats the point of the lazy-loading entirely.

Steps

• Fix where lazy loading is being silently negated (e.g., eager evaluation of getters, un-awaited fills of extended attributes)
• Concurrency safety (e.g., locking resolution of association, infinite-loop detection, guarding modification of shared lists)
• Deterministic collection ordering (replacing completion-order with source-order)
• ReverseTopicMappingService:
  • Fix a data-integrity bug where clearing relationships doesn't purge deferred entries, so deletions get silently resurrected
  • Sequentializing its fan-out to avoid concurrent writes to the same collections on the target Topic