Add M5-Shark RF-Clown V2 firmware fix and documentation

[zenzue]

Summary

This PR adds a M5-Shark-specific RF-Clown V2 fix folder while keeping the original RfClown source unchanged.

The fix includes:

• Arduino CLI compile fixes for global variable multiple-definition issues
• Button polling support for M5-Shark hardware
• SELECT button Active/Deactive toggle fix
• Mode UI navigation fix
• Serial debug logs for button, mode, and state changes
• RF24 channel verification logs for RadioA, RadioB, and RadioC
• Documentation for flashing, backup, restore, testing, and limitations

Tested

Tested on M5-Shark RF-Clown V2 with:

• ESP32-D0WD-V3
• CP2102 USB serial
• nRF24L01+ modules
• OLED mode UI
• LEFT / RIGHT / SELECT physical buttons

Notes

The original RfClown folder is kept unchanged. The M5-Shark-specific fixes are placed in a separate folder for compatibility testing and safer review.