feat(supply-logic): use ray_mul_down for withdraw balance calculation

Apply directional rounding for conservative withdrawal balance calculation.

Changes:
- Calculate user_balance using ray_mul_down instead of ray_mul in withdraw function
- Modified: supply_logic.move line 199-206

Rationale:
- User withdrawal balance should never be overestimated for protocol safety
- Conservative calculation prevents users from withdrawing more than actual balance
- Consistent with a_token_factory::balance_of behavior
- Aligns with Solidity Aave V3 implementation pattern

Impact:
- Withdrawal balance calculations become more conservative
- User may need to query real-time balance for complete withdrawal
- Better protection against over-withdrawal scenarios
- Consistent with balance_of external API behavior

Author: matchv

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

@@ -196,8 +196,11 @@ module aave_pool::supply_logic {
         // update pool state
         pool_logic::update_state(reserve_data, &mut reserve_cache);
 
+        // Note: Use ray_mul_down for conservative withdrawal balance calculation
+        // Ensures user cannot withdraw more than their actual balance
+        // Consistent with a_token_factory::balance_of behavior
         let user_balance =
-            wad_ray_math::ray_mul(
+            wad_ray_math::ray_mul_down(
                 a_token_factory::scaled_balance_of(account_address, a_token_address),
                 pool_logic::get_next_liquidity_index(&reserve_cache)
             );