Fix 8 bugs from code review

B#1 — sigCh shared across goroutines, SIGINT unreliable
  → context.WithCancel: main loop cancels ctx on SIGINT,
    4 WS goroutines select on ctx.Done() instead of shared sigCh

B#3 — restoreOpenPositions missing LastScaleAt
  → Set LastScaleAt = tr.OpenedAt on restore so scale-in cooldown works

B#4 — dYdX heartbeat goroutine leaks on reconnect
  → Added stopHeartbeat chan + heartbeatMu mutex; close old channel
    before spawning new heartbeat goroutine

B#5 — GetBitgetSize fmt.Sprintf rounds up, may exceed amountUSD
  → Added math.Floor(sz*multiplier)/multiplier before format to round
    DOWN to nearest valid step size for every coin

B#6 — netProfit and CalcNetProfit duplicate formula
  → scanner.go netProfit now delegates to exchange.CalcNetProfit

B#7 — Aevo Run callback only 2 params, incompatible with startExchange
  → Changed to 4-arg callback func(coin, price, bid, ask) with bid=ask=0

B#8 — parseFloat uses fmt.Sscanf (slow, locale-sensitive)
  → Replaced with strconv.ParseFloat

B#9 — dYdX receives hlSymbols instead of its own symbol list
  → Added dydxSymbols var, built from c.HL like other exchanges

exchange/bitget_trade.go

@@ -7,6 +7,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"math"
 	"net/http"
 	"strings"
 	"time"
@@ -95,6 +96,7 @@ func (b *BitgetTrade) sign(method, requestPath, timestamp, body string) string {
 // GetBitgetSize calculates the contract size for a given USD amount.
 // Returns size as a decimal string complying with Bitget's USDT-FUTURES precision.
 // Enforces the exchange's minimum: minTradeNum contracts AND $5 min notional.
+// Uses math.Floor to round DOWN to the nearest valid step (B#5: prevent rounding up).
 func GetBitgetSize(symbol string, amountUSD, price float64) string {
 	if amountUSD < 5 {
 		amountUSD = 5 // Bitget minimum notional
@@ -106,28 +108,34 @@ func GetBitgetSize(symbol string, amountUSD, price float64) string {
 		if sz < 1 {
 			sz = 1
 		}
+		sz = math.Floor(sz) // step=1
 		return fmt.Sprintf("%.0f", sz) // minTradeNum=1, sizeMultiplier=1
 	case "LINKUSDT":
 		if sz < 1 {
 			sz = 1
 		}
+		sz = math.Floor(sz) // step=1
 		return fmt.Sprintf("%.0f", sz) // minTradeNum=1, sizeMultiplier=1
 	case "ONDOUSDT":
+		sz = math.Floor(sz*10) / 10 // step=0.1
 		if sz < 0.1 {
 			sz = 0.1
 		}
 		return fmt.Sprintf("%.1f", sz) // minTradeNum=0.1, sizeMultiplier=0.1
 	case "OPUSDT":
+		sz = math.Floor(sz*10) / 10 // step=0.1
 		if sz < 0.1 {
 			sz = 0.1
 		}
 		return fmt.Sprintf("%.1f", sz) // minTradeNum=0.1, sizeMultiplier=0.1
 	case "WIFUSDT":
+		sz = math.Floor(sz*10) / 10 // step=0.1
 		if sz < 0.1 {
 			sz = 0.1
 		}
 		return fmt.Sprintf("%.1f", sz) // minTradeNum=0.1, sizeMultiplier=0.1
 	case "ARBUSDT":
+		sz = math.Floor(sz*100) / 100 // step=0.01
 		if sz < 0.01 {
 			sz = 0.01
 		}