radio_init() configures RADIO in NRF_1Mbit proprietary mode: 8-bit
length field, 4-byte address (BASE0+PREFIX0), 2-byte CRC-16/CCITT,
0 dBm TX power, channel 20 (2420 MHz). READY->START and END->DISABLE
shortcuts let radio_tx() trigger the full ramp-up/TX/disable sequence
by writing TASKS_TXEN once, then polling EVENTS_END.
main.c sends a fixed 4-byte test frame on every button press.
FHSS and AES remain compiled but are not called.
New bitfield unions in regs.h: radio_pcnf0_t, radio_pcnf1_t,
radio_crccnf_t, radio_shorts_t.
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