fix: simplify interest reset in deductFixedPositionDebt during liquidation

Remove partial interest tracking logic and always reset interestRepaid
and lastRepaidTime after liquidation deduction.

Author: ricklista

src/broker/libraries/BrokerMath.sol

@@ -546,27 +546,10 @@ library BrokerMath {
       // update repaid principal amount
       principalToDeduct -= repayPrincipalAmt;
       p.principalRepaid += repayPrincipalAmt;
-
-      if (repayInterestAmt >= accruedInterest) {
-        // all accrued interest fully covered -> safe to reset tracking
-        p.interestRepaid = 0;
-        p.lastRepaidTime = block.timestamp;
-      } else {
-        // partial interest covered -> adjust interestRepaid to preserve outstanding
-        // After principalRepaid increased, getAccruedInterestForFixedPosition() recalculates
-        // with smaller (principal - principalRepaid), producing a lower total (newTotalAccrued).
-        // We set interestRepaid so that: newTotalAccrued - interestRepaid = unpaidInterest
-        uint256 unpaidInterest = accruedInterest - repayInterestAmt;
-        uint256 newTotalAccrued = getAccruedInterestForFixedPosition(p);
-        if (newTotalAccrued >= unpaidInterest) {
-          // exact: outstanding is perfectly preserved
-          p.interestRepaid = newTotalAccrued - unpaidInterest;
-        } else {
-          // edge case: most principal repaid, formula can't represent full outstanding
-          // preserve maximum possible: outstanding = newTotalAccrued (don't reset lastRepaidTime)
-          p.interestRepaid = 0;
-        }
-      }
+      // reset repaid interest to zero (all accrued interest has been cleared)
+      p.interestRepaid = 0;
+      // reset repaid time to now
+      p.lastRepaidTime = block.timestamp;
     }
 
     return (interestToDeduct, principalToDeduct, p);