ESPNOW wireless trainer mode

[robustini]

Adds wireless trainer (buddy box) support to the TX backpack using ESPNOW. No cable required between master and slave radio.

Works in conjunction with ExpressLRS TX PR #3630.

Modes

Master: receives RC channels from the slave backpack via ESPNOW and injects them into EdgeTX as a standard CRSF trainer input

Slave: reads RC channels from EdgeTX and forwards them to the master backpack via ESPNOW at 50 Hz

Setup

• Open the ELRS Lua script on both radios
• Navigate to Backpack and set Telemetry to ESPNOW on both radios
• Set Trainer Mode to Master on the teacher radio
• Set Trainer Mode to Slave on the student radio
• On both radios, press Pair Trainer within the 30-second pairing window
• Wait until both sides report Paired:XX
• On the master radio, set Trainer to CRSF in EdgeTX Model Settings and assign a switch to enable trainer input as usual

Trainer Mode, Pair Trainer, Forget Pair and the trainer status are available from the Backpack page when Telemetry is set to ESPNOW. The trainer status shows Not paired, Pairing... or Paired:XX. Pairing now requires both radios to explicitly enter Pair Trainer mode within the timeout window and is independent from the ELRS bind phrase / UID. Once paired, the link is restored automatically on every boot without re-pairing.