Fix ESP-NOW unicast by using sender MAC in client registry.

Register slaves from recv src_addr instead of protobuf mac bytes, add
ESPNOW_UNICAST_TEST for path verification, restore unicast deadzone, and
expose unicast-test in goTool.

Co-authored-by: Cursor <cursoragent@cursor.com>

goTool/main.go

@@ -14,7 +14,8 @@ func usage() {
 	fmt.Fprintf(os.Stderr, "commands:\n")
 	fmt.Fprintf(os.Stderr, "  version   firmware version and git hash\n")
 	fmt.Fprintf(os.Stderr, "  clients   registered ESP-NOW slaves on the master\n")
-	fmt.Fprintf(os.Stderr, "  deadzone  get/set accelerometer deadzone (LSB)\n\n")
+	fmt.Fprintf(os.Stderr, "  deadzone      get/set accelerometer deadzone (LSB)\n")
+	fmt.Fprintf(os.Stderr, "  unicast-test  send ESP-NOW unicast test to one slave\n\n")
 	flag.PrintDefaults()
 }
 
@@ -44,6 +45,8 @@ func main() {
 		runErr = runClients(sp)
 	case "deadzone", "accel-deadzone":
 		runErr = runDeadzone(sp, flag.Args()[1:])
+	case "unicast-test", "unicast_test":
+		runErr = runUnicastTest(sp, flag.Args()[1:])
 	default:
 		fmt.Fprintf(os.Stderr, "unknown command %q\n\n", cmd)
 		usage()