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/cmd_unicast.go

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+
+	"google.golang.org/protobuf/proto"
+
+	"powerpod/gotool/pb"
+)
+
+func runUnicastTest(sp *serialPort, args []string) error {
+	fs := flag.NewFlagSet("unicast-test", flag.ExitOnError)
+	clientID := fs.Uint("client", 0, "slave client id from `clients`")
+	seq := fs.Uint("seq", 1, "test sequence number")
+	if err := fs.Parse(args); err != nil {
+		return err
+	}
+	if *clientID == 0 {
+		return fmt.Errorf("client id required (see `gotool clients`)")
+	}
+
+	req := &pb.EspNowUnicastTestRequest{
+		ClientId: uint32(*clientID),
+		Seq:      uint32(*seq),
+	}
+	msg := &pb.UartMessage{
+		Type: pb.MessageType_ESPNOW_UNICAST_TEST,
+		Payload: &pb.UartMessage_EspnowUnicastTestRequest{
+			EspnowUnicastTestRequest: req,
+		},
+	}
+	body, err := proto.Marshal(msg)
+	if err != nil {
+		return fmt.Errorf("encode request: %w", err)
+	}
+
+	payload := append([]byte{byte(pb.MessageType_ESPNOW_UNICAST_TEST)}, body...)
+	respPayload, err := sp.exchangePayload(payload, "ESPNOW_UNICAST_TEST")
+	if err != nil {
+		return err
+	}
+
+	var respMsg pb.UartMessage
+	if err := proto.Unmarshal(respPayload[1:], &respMsg); err != nil {
+		return fmt.Errorf("decode response: %w", err)
+	}
+
+	r := respMsg.GetEspnowUnicastTestResponse()
+	if r == nil {
+		return fmt.Errorf("response missing espnow_unicast_test_response")
+	}
+
+	fmt.Printf("unicast test sent: success=%v seq=%d\n", r.GetSuccess(), r.GetSeq())
+	return nil
+}