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>

main/proto/uart_messages.proto

@@ -10,6 +10,7 @@ enum MessageType {
   CLIENT_INFO = 4;
   CLIENT_INPUT = 5;
   ACCEL_DEADZONE = 6;
+  ESPNOW_UNICAST_TEST = 7;
   OTA_START = 16;
   OTA_PAYLOAD = 17;
   OTA_END = 18;
@@ -31,6 +32,8 @@ message UartMessage {
     OtaStatusPayload ota_status = 10;
     AccelDeadzoneRequest accel_deadzone_request = 11;
     AccelDeadzoneResponse accel_deadzone_response = 12;
+    EspNowUnicastTestRequest espnow_unicast_test_request = 13;
+    EspNowUnicastTestResponse espnow_unicast_test_response = 14;
   }
 }
 
@@ -87,6 +90,16 @@ message AccelDeadzoneResponse {
   uint32 slaves_updated = 4;
 }
 
+message EspNowUnicastTestRequest {
+  uint32 client_id = 1;
+  uint32 seq = 2;
+}
+
+message EspNowUnicastTestResponse {
+  bool success = 1;
+  uint32 seq = 2;
+}
+
 message OtaStartPayload {
   uint32 total_size = 1;
   uint32 block_size = 2;