Ensure HashSetBase::Compact() preserves _elementsCount even when EnsureCapacity() triggers during compaction. The growth path resets the counter; we now cache the original count and restore it after moving all buckets so Count() stays correct in heavy-collision scenarios.
- Compact now iterates over the old bucket array using the saved oldSize, and frees with that size, avoiding out-of-bounds when _size changes.
- If reinsertion finds no free slot during compaction (pathological collisions), the table grows once and retries, preventing AVs.
- This fix addresses problems with weak hash keys (like #3824).
