Problem:
- Previous logic: push_back() first, then pop_front() if len > 10
- This temporarily creates 11 elements, exceeding VecDeque capacity of 10
- Wastes the benefit of with_capacity() pre-allocation
Solution:
- Check capacity BEFORE adding: if len >= 10, pop_front() first
- Then push_back() new element
- Guarantees never exceeding 10 elements at any time
Changes:
- mark_block_downloaded(): inverted order (pop before push)
- Changed condition from `> CACHE_SIZE` to `>= CACHE_SIZE`
- Documentation updated with correct logic and benefits
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