Initial bare-metal foundation for nRF52840 PTT-FHSS

- Fully open toolchain: arm-none-eabi-gcc inside Podman/Docker container
- Platform-agnostic build via justfile (auto-detects podman vs docker)
- CMake + Ninja build system with arm-none-eabi toolchain file
- nRF52840 linker script and startup with full 64-entry vector table
- Register access via typed bitfield unions (include/regs.h)
- FHSS channel sequencer: AES-128-ECB PRNG over 40 channels (2402-2480 MHz)
- Low-power sleep via SYSTEM_ON WFI + GPIOTE wakeup on button press
- Vendor deps as shallow git submodules: nrfx, CMSIS_5, tiny-AES-c

include/radio.h

@@ -0,0 +1,7 @@
+#pragma once
+#include <stdint.h>
+
+void    radio_init(void);
+void    radio_set_channel(uint8_t ch);        /* 0–39, maps to 2402–2480 MHz */
+void    radio_tx(const uint8_t *data, uint8_t len);
+void    radio_tx_burst(void);                 /* TX with FHSS hopping */