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

prisma/schema.prisma

@@ -109,6 +109,9 @@ model Trade {
   status           String   @default("open") // "open", "closed", "failed", "phantom"
   isTestTrade      Boolean  @default(false) // Flag test trades for exclusion from analytics
   
+  // Fractional remnant tracking (Bug #89 - Dec 16, 2025)
+  closeAttempts    Int?     // Number of close attempts (for fractional remnant detection)
+  
   // Phantom trade detection
   isPhantom        Boolean  @default(false) // Position opened but size mismatch >50%
   expectedSizeUSD  Float?   // Expected position size (when phantom)