Slaves send HEARTBEAT every 1s; the master marks clients inactive after
3s without traffic and reactivates on reconnect. CLIENT_INFO reports
last_ping as milliseconds since the last packet, not uptime.
Co-authored-by: Cursor <cursoragent@cursor.com>
Track ESP-NOW slaves in a shared registry and respond to CLIENT_INFO
with protobuf ClientInfoResponse; ESP-NOW path upserts registry entries.
Co-authored-by: Cursor <cursoragent@cursor.com>
