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/aevo.go

@@ -47,7 +47,7 @@ func NewAevoWS(tracked []TrackedSymbol) *AevoWS {
 }
 
 // Run connects to Aevo WS and streams ticker data.
-func (a *AevoWS) Run(updateFn func(coin string, price float64)) error {
+func (a *AevoWS) Run(updateFn func(coin string, price, bid, ask float64)) error {
 	a.Conn.OnConnect = func() {
 		log.Printf("[Aevo WS] Connected, subscribing to %d tickers", len(a.Tracked))
 
@@ -120,7 +120,7 @@ func (a *AevoWS) Run(updateFn func(coin string, price float64)) error {
 			}
 
 			if price > 0 {
-				updateFn(coin, price)
+				updateFn(coin, price, 0, 0) // B#7: pass bid=ask=0 for 4-arg signature
 			}
 		}
 	}