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/README.md

@@ -24,9 +24,16 @@ go run . -port /dev/ttyUSB0 clients
 |---------|--------------|-------------|
 | `version` | `0x03` | Prints `version` and `git_hash` from firmware |
 | `clients` | `0x04` | Lists slaves registered on the master via ESP-NOW |
+| `unicast-test` | `0x07` | Sends ESP-NOW unicast test to one slave (`-client`, `-seq`) |
 
 `clients` requires slaves to have responded to master discover broadcasts first.
 
+```bash
+go run . -port /dev/ttyUSB0 unicast-test -client 16 -seq 42
+```
+
+On success the slave serial log should show `UNICAST TEST OK from master … seq=42`.
+
 Example output:
 
 ```