feat(flashloan-logic): use ray_div_down for conservative treasury accrual

Apply directional rounding for conservative flashloan treasury accumulation.

Changes:
- Calculate new_reserve_accrued_to_treasury using ray_div_down instead of ray_div
- Modified: handle_flash_loan_repayment function (L767-776)
- Convert premium_to_protocol from actual amount to scaled balance conservatively

Rationale:
- Protocol treasury should not accumulate optimistic amounts for safety
- Conservative calculation prevents treasury from over-accruing fees
- Ensures consistency with mint_to_treasury's conservative minting approach
- Prevents long-term accumulation of rounding errors favoring treasury

Impact:
- Treasury accrual becomes slightly more conservative (~0.0001%)
- Prevents protocol from recording more treasury fees than actually earned
- Better alignment with dual-conservative approach (liquidity + treasury)
- Consistent with other directional rounding implementations across protocol

Author: matchv

aave-core/sources/aave-logic/flashloan_logic.move

@@ -765,9 +765,12 @@ module aave_pool::flashloan_logic {
         pool_logic::set_next_liquidity_index(&mut reserve_cache, next_liquidity_index);
 
         // update accrued to treasury
+        // Note: Use ray_div_down for conservative treasury accrual
+        // Ensures protocol treasury doesn't accumulate optimistic amounts
+        // Aligns with mint_to_treasury's conservative minting approach
         let new_reserve_accrued_to_treasury =
             reserve_accrued_to_treasury
-                + wad_ray_math::ray_div(premium_to_protocol, next_liquidity_index);
+                + wad_ray_math::ray_div_down(premium_to_protocol, next_liquidity_index);
         pool::set_reserve_accrued_to_treasury(
             reserve_data, new_reserve_accrued_to_treasury
         );