critical: Bug #89 - Detect and handle Drift fractional position remnants (3-part fix)

- Part 1: Position Manager fractional remnant detection after close attempts
  * Check if position < 1.5× minOrderSize after close transaction
  * Log to persistent logger with FRACTIONAL_REMNANT_DETECTED
  * Track closeAttempts, limit to 3 maximum
  * Mark exitReason='FRACTIONAL_REMNANT' in database
  * Remove from monitoring after 3 failed attempts

- Part 2: Pre-close validation in closePosition()
  * Check if position viable before attempting close
  * Reject positions < 1.5× minOrderSize with specific error
  * Prevent wasted transaction attempts on too-small positions
  * Return POSITION_TOO_SMALL_TO_CLOSE error with manual instructions

- Part 3: Health monitor detection for fractional remnants
  * Query Trade table for FRACTIONAL_REMNANT exits in last 24h
  * Alert operators with position details and manual cleanup instructions
  * Provide trade IDs, symbols, and Drift UI link

- Database schema: Added closeAttempts Int? field to Track attempts

Root cause: Drift protocol exchange constraints can leave fractional positions
Evidence: 3 close transactions confirmed but 0.15 SOL remnant persisted
Financial impact: ,000+ risk from unprotected fractional positions
Status: Fix implemented, awaiting deployment verification

See: docs/COMMON_PITFALLS.md Bug #89 for complete incident details

lib/drift/orders.ts

@@ -563,6 +563,29 @@ export async function closePosition(
         error: 'No open position to close',
       }
     }
+    
+    // BUG #89 FIX PART 2 (Dec 16, 2025): Pre-close validation for fractional positions
+    // Before attempting close, check if position is too small to close reliably
+    // Drift protocol has exchange-level constraints that prevent closing very small positions
+    // If position below minimum viable size, return error with manual resolution instructions
+    const oraclePriceForCheck = await driftService.getOraclePrice(marketConfig.driftMarketIndex)
+    const positionSizeUSD = Math.abs(position.size) * oraclePriceForCheck
+    const minViableSize = marketConfig.minOrderSize * 1.5
+    
+    if (positionSizeUSD < minViableSize && params.percentToClose >= 100) {
+      console.log(`🛑 POSITION TOO SMALL TO CLOSE`)
+      console.log(`   Position: ${Math.abs(position.size)} tokens ($${positionSizeUSD.toFixed(2)})`)
+      console.log(`   Minimum viable size: $${minViableSize.toFixed(2)}`)
+      console.log(`   This position is below Drift protocol's minimum viable close size`)
+      console.log(`   Close transactions may confirm but leave fractional remnants`)
+      console.log(`   Manual intervention required via Drift UI`)
+      
+      return {
+        success: false,
+        error: 'POSITION_TOO_SMALL_TO_CLOSE',
+        closedSize: Math.abs(position.size),
+      } as any
+    }
 
     logger.log('📊 Closing position:', params)
     console.log(`   params.percentToClose: ${params.percentToClose}`)