critical: Bug #91 - Use SDK closePosition() for exact BN (no Math.floor truncation)

Root Cause: Math.floor(sizeToClose * 1e9) truncated position sizes, leaving tiny fractional remnants (e.g., 0.00000008 SOL) that prevented full position closure.

Discovery: Drift UI 'Close All Positions' failed with 'not enough collateral' but clicking 'Market' order worked - because Market uses exact position size.

Solution: SDK's driftClient.closePosition() uses exact BN arithmetic internally (baseAssetAmount.abs()), avoiding any floating point truncation.

Changes:
- lib/drift/orders.ts lines 647-690
- For 100% closes: Now uses driftClient.closePosition(marketIndex)
- For partial closes: Continues using placeAndTakePerpOrder

Expected Impact: Flip operations will now fully close positions without leaving fractional remnants that cause 'position still open' failures.

Financial Impact: Prevents flip failures that caused user 000+ losses from multiple bugs in position closing logic.

lib/drift/orders.ts

@@ -644,35 +644,48 @@ export async function closePosition(
       }
     }
     
-    // Prepare close order (opposite direction)
-    const orderParams = {
-      orderType: OrderType.MARKET,
-      marketIndex: marketConfig.driftMarketIndex,
-      direction: position.side === 'long' 
-        ? PositionDirection.SHORT 
-        : PositionDirection.LONG,
-      baseAssetAmount: new BN(Math.floor(sizeToClose * 1e9)), // 9 decimals
-      reduceOnly: true, // Important: only close existing position
-    }
+    // CRITICAL FIX (Jan 1, 2026 - Bug #91): Use SDK's closePosition() for 100% closes
+    //
+    // ROOT CAUSE: Our baseAssetAmount calculation uses:
+    //   baseAssetAmount: new BN(Math.floor(sizeToClose * 1e9))
+    // 
+    // The Math.floor() TRUNCATES, leaving fractional remnants that never close!
+    // Example: Drift has 23.02145678 SOL, we try to close 23.02145670 SOL
+    //          Remaining 0.00000008 SOL stays open forever!
+    //
+    // SOLUTION: For 100% closes, use SDK's driftClient.closePosition() which:
+    //   1. Gets EXACT position size from Drift as BN (no floating point)
+    //   2. Uses baseAssetAmount.abs() internally (no truncation)
+    //   3. Calls placeAndTakePerpOrder with exact amount
+    //
+    // This matches what the Drift UI "Market" button does vs "Close All" which
+    // may use a similar size calculation that leaves remnants.
     
-    // CRITICAL FIX (Dec 31, 2025 - Bug #89): Use placeAndTakePerpOrder instead of placePerpOrder
-    // 
-    // ROOT CAUSE: placePerpOrder only PLACES an order on the order book.
-    // For MARKET orders, the transaction confirms when the order is placed, NOT when it's filled.
-    // This caused the flip operation to fail - close tx confirmed but position never closed.
-    // 
-    // SOLUTION: placeAndTakePerpOrder places the order AND fills it against makers atomically.
-    // This guarantees the position is closed when the transaction confirms.
-    //
-    // Evidence: Solscan showed tx 5E713pD6... as "Place long 24.72 SOL perp order at price 0"
-    // - The order was placed on the book but never filled
-    // - Position remained open despite "confirmed" transaction
-    //
-    // See: https://github.com/drift-labs/drift-vaults/blob/main/tests/testHelpers.ts#L1542
-    console.log('🚀 Placing market close order with IMMEDIATE FILL (placeAndTakePerpOrder)...')
-    const txSig = await retryWithBackoff(async () => {
-      return await driftClient.placeAndTakePerpOrder(orderParams)
-    }, 3, 8000) // 8s base delay, 3 max retries
+    let txSig: string
+    
+    if (params.percentToClose === 100) {
+      // USE SDK'S BUILT-IN CLOSE - Gets EXACT position size from Drift internally
+      console.log('🚀 Using SDK closePosition() for 100% close (exact BN, no truncation)...')
+      txSig = await retryWithBackoff(async () => {
+        return await driftClient.closePosition(marketConfig.driftMarketIndex)
+      }, 3, 8000) // 8s base delay, 3 max retries
+    } else {
+      // Partial close - use our calculation (partial closes are less critical)
+      const orderParams = {
+        orderType: OrderType.MARKET,
+        marketIndex: marketConfig.driftMarketIndex,
+        direction: position.side === 'long' 
+          ? PositionDirection.SHORT 
+          : PositionDirection.LONG,
+        baseAssetAmount: new BN(Math.floor(sizeToClose * 1e9)), // 9 decimals
+        reduceOnly: true,
+      }
+      
+      console.log('🚀 Placing partial close order with placeAndTakePerpOrder...')
+      txSig = await retryWithBackoff(async () => {
+        return await driftClient.placeAndTakePerpOrder(orderParams)
+      }, 3, 8000)
+    }
     
     console.log(`✅ Close order executed! Transaction: ${txSig}`)