Profiling Results:
- Fast path: 143 cycles (10.4% of time) ✅ Good
- Refill: 19,624 cycles (89.6% of time) 🚨 Bottleneck!
Refill is 137x slower than fast path and dominates total cost.
Only happens 6.3% of the time but takes 90% of execution time.
Next: Optimize sll_refill_small_from_ss() backend.