fix: HL testnet - add API wallet address to signed orders, 3-param constructor

config.go

@@ -56,7 +56,8 @@ type Config struct {
 
 	// HyperLiquid API
 	HLPrivateKey string // ed25519 private key hex
-	HLAddress    string // wallet address
+	HLAddress    string // main account address
+	HLAPIAddress string // API wallet address (signer, auto-derived if empty)
 }
 
 // jsonConfig maps config.json fields (non-secret defaults checked into git).
@@ -157,6 +158,7 @@ func LoadConfig() *Config {
 
 		HLPrivateKey: getEnv("HL_PRIVATE_KEY", ""),
 		HLAddress:    getEnv("HL_ADDRESS", ""),
+		HLAPIAddress: getEnv("HL_API_ADDRESS", ""),
 	}
 }