Synchronize ESPNOWRadio RX frame handoff

[robekl]

Summary

This fixes the ESP-NOW radio receive handoff so callback context no longer races the main loop over a single shared RX buffer.

Problem

ESPNOWRadio currently receives frames like this:

• OnDataRecv() copies bytes into one global rx_buf
• it then publishes last_rx_len
• recvRaw() later reads last_rx_len, copies from that same buffer, and clears the length

There is no atomic handoff, no critical section, and no second buffer. If a callback lands while recvRaw() is reading, the consumer can observe torn data, lose a packet, or read a length that no longer matches the payload currently in the buffer.

Fix

This PR replaces the single shared RX slot with a small fixed-size mailbox queue:

• the ESP-NOW callback now enqueues complete received frames into a bounded ring buffer
• queue publication/consumption is protected by a very short critical section
• recvRaw() pops one complete frame atomically and only then copies it into the caller buffer
• explicit overflow logging/drop counting is added instead of silent overwrite

I also added the small no-op compatibility helpers currently expected by the ESP-NOW companion target (getPacketsRecvErrors(), setRxBoostedGainMode(), powerOff()) so the intended build target stays green.

Why this fixes it

The producer and consumer no longer share one mutable rx_buf/last_rx_len pair. A received frame is either fully published in a queue slot or not visible yet, and recvRaw() only consumes stable slots after claiming them under lock.

That removes the callback/main-loop race that previously allowed partial or corrupted reads under load.

Validation

Built successfully:

• SenseCapIndicator-ESPNow_comp_radio_usb