Added ECHO cmd and fixed some timing issues

goTool/README.md

@@ -28,6 +28,7 @@ go run . -port /dev/ttyUSB0 clients
 | `tap-notify` | `0x1b` | Get/set which tap kinds (single/double/triple) notify via ESP-NOW (`-set`, `-client`, `-all`, `-single`, `-double`, `-triple`) |
 | `cache-status` | `0x1d` | Subscribed accel + tap cache (`CACHE_STATUS`); one UART round-trip for 16 ms polling |
 | `unicast-test` | `0x07` | Sends ESP-NOW unicast test to one slave (`-client`, `-seq`) |
+| `echo-ping` | `0x1e` | ESP-NOW echo round-trip to one slave (`-client`); prints `rtt_ms` (host UART chain) and `esp_rtt_us` (master ESP-NOW, raw µs) |
 | `test` | — | Run an automated scenario (JSON configs under `testdata/`) |
 | `serve` | — | Web dashboard at `http://localhost:8080` (WebSocket live updates) |
 | `ota` | 16–19 | UART firmware upload to master; firmware then pushes to slaves via ESP-NOW |
@@ -113,8 +114,20 @@ 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`.
 
+```bash
+go run . -port /dev/ttyUSB0 echo-ping -client 16
+```
+
+Measures latency to one slave. `rtt_ms` is the full host round-trip (UART + ESP-NOW + UART back). `esp_rtt_us` is the master-side ESP-NOW leg only (`esp_timer_get_time()` delta, raw microseconds from firmware).
+
 Example output:
 
+```
+echo ping: success=true client_id=16 rtt_ms=49.729 esp_rtt_us=18234
+```
+
+`clients` example:
+
 ```
 clients (2):
   [0] id=42 mac=aabbccddeeff ver=1 available=true used=false last_ping=250 last_success_ping=250