Problem:
- With `if >= CACHE_SIZE`, only ONE element removed per call
- If cache ever had >10 elements (abnormal state), would stay oversized
- Example: 12 elements → if removes 1 → 11 elements → add 1 → 12 elements ❌
Solution:
- Use `while >= CACHE_SIZE` to remove ALL excess elements
- Example: 12 elements → while removes 2 → 10 elements → add 1 → 10 elements ✅
- Guarantees exactly ≤10 elements regardless of initial state
Changes:
- mark_block_downloaded(): changed `if` to `while`
- Updated comment to reflect "tous les éléments excédentaires"
- Documentation updated with robustness guarantee
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