fix: HL size floor, random scan jitter 50-250ms

- GetHLSize: change rounding from Sprintf (round-to-nearest)
  to math.Floor (floor), consistent with GetBitgetSize
- Scan interval: fixed 200ms → random 50-250ms to avoid
  lock-step with HyperLiquid's ~200ms allMids push cycle
- README: update architecture diagram, trading logic, config note

exchange/hyperliquid_trade.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"math"
 	"math/big"
 	"net/http"
 	"strings"
@@ -171,21 +172,28 @@ func (h *HyperLiquidTrade) signAction(action HLOrderAction, nonce int64) (string
 // GetHLSize calculates size for a given USD amount on HyperLiquid.
 // Uses szDecimals precision from HL's contract universe.
 // Returns size as a decimal string complying with HL precision.
+// Uses math.Floor to round DOWN to the nearest valid step, consistent with GetBitgetSize.
 func GetHLSize(coin string, amountUSD, price float64) string {
 	sz := amountUSD / price // raw coin count
 	switch coin {
 	case "DOGE":
-		return fmt.Sprintf("%.0f", sz) // szDecimals=0
+		sz = math.Floor(sz) // step=1, szDecimals=0
+		return fmt.Sprintf("%.0f", sz)
 	case "LINK":
-		return fmt.Sprintf("%.1f", sz) // szDecimals=1
+		sz = math.Floor(sz*10) / 10 // step=0.1, szDecimals=1
+		return fmt.Sprintf("%.1f", sz)
 	case "ONDO":
-		return fmt.Sprintf("%.0f", sz) // szDecimals=0
+		sz = math.Floor(sz) // step=1, szDecimals=0
+		return fmt.Sprintf("%.0f", sz)
 	case "OP":
-		return fmt.Sprintf("%.1f", sz) // szDecimals=1
+		sz = math.Floor(sz*10) / 10 // step=0.1, szDecimals=1
+		return fmt.Sprintf("%.1f", sz)
 	case "WIF":
-		return fmt.Sprintf("%.0f", sz) // szDecimals=0
+		sz = math.Floor(sz) // step=1, szDecimals=0
+		return fmt.Sprintf("%.0f", sz)
 	case "ARB":
-		return fmt.Sprintf("%.1f", sz) // szDecimals=1
+		sz = math.Floor(sz*10) / 10 // step=0.1, szDecimals=1
+		return fmt.Sprintf("%.1f", sz)
 	default:
 		return fmt.Sprintf("%.4f", sz)
 	}