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

.gitmodules

@@ -0,0 +1,9 @@
+[submodule "vendor/nrfx"]
+	path = vendor/nrfx
+	url = https://github.com/NordicSemiconductor/nrfx.git
+[submodule "vendor/CMSIS_5"]
+	path = vendor/CMSIS_5
+	url = https://github.com/ARM-software/CMSIS_5.git
+[submodule "vendor/tiny-aes-c"]
+	path = vendor/tiny-aes-c
+	url = https://github.com/kokke/tiny-AES-c.git