Add SpreadWindowTracker to measure opportunity duration

- New SpreadWindowTracker in types.go watches BG↔HL spread for all
  tracked coins, both directions
- Logs duration when spread stays above trade threshold then converges
- Wired into main loop after each scan tick
- Filters sub-100ms windows as noise

main.go

@@ -51,6 +51,9 @@ func main() {
 	dashboard := NewDashboard(store, trader, database, ":8888")
 	go dashboard.Run()
 
+	// Spread window tracker — measures how long spreads stay above threshold
+	spreadTracker := NewSpreadWindowTracker()
+
 	// P3-4: wire real-time trade event broadcast
 	trader.OnTradeEvent = dashboard.BroadcastEvent
 	if trader.IsConfigured() {
@@ -170,6 +173,10 @@ func main() {
 			dashboard.UpdateScan(makerOpps)
 			t2 := time.Now()
 
+			// Track spread window durations (how long each opportunity stays alive)
+			snap := store.GetAll()
+			spreadTracker.Tick(snap, cfg.TradeThreshold)
+
 			for _, opp := range makerOpps {
 				if opp.NetProfit < cfg.ArbThreshold {
 					continue