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

CMakeLists.txt

@@ -0,0 +1,71 @@
+cmake_minimum_required(VERSION 3.22)
+
+# must be set before project()
+set(CMAKE_TOOLCHAIN_FILE "${CMAKE_SOURCE_DIR}/cmake/arm-none-eabi.cmake")
+
+project(ptt_fhss C ASM)
+
+set(CMAKE_C_STANDARD 11)
+set(CMAKE_C_STANDARD_REQUIRED ON)
+
+# sources
+add_executable(firmware
+    src/startup.c
+    src/main.c
+    src/radio.c
+    src/fhss.c
+    src/power.c
+    vendor/tiny-aes-c/aes.c
+)
+
+# include paths
+target_include_directories(firmware PRIVATE
+    include
+    vendor/nrfx/mdk                    # nRF52840 device headers (nrf52840.h etc.)
+    vendor/CMSIS_5/CMSIS/Core/Include  # core_cm4.h, cmsis_gcc.h etc.
+    vendor/tiny-aes-c
+)
+
+# preprocessor defines
+target_compile_definitions(firmware PRIVATE
+    NRF52840_XXAA  # device variant required by nrfx/mdk headers
+)
+
+# compiler flags
+set(CPU_FLAGS
+    -mcpu=cortex-m4
+    -mthumb
+    -mfpu=fpv4-sp-d16
+    -mfloat-abi=hard
+)
+
+target_compile_options(firmware PRIVATE
+    ${CPU_FLAGS}
+    -Os
+    -ffunction-sections
+    -fdata-sections
+    -fno-exceptions
+    -Wall
+    -Wextra
+    -Werror
+)
+
+# linker
+target_link_options(firmware PRIVATE
+    ${CPU_FLAGS}
+    -T ${CMAKE_SOURCE_DIR}/link/nrf52840.ld
+    -Wl,--gc-sections
+    -Wl,--print-memory-usage
+    -nostartfiles
+    -specs=nano.specs
+    -specs=nosys.specs
+)
+
+# post-build: .hex + size report
+add_custom_command(TARGET firmware POST_BUILD
+    COMMAND arm-none-eabi-objcopy
+            -O ihex $<TARGET_FILE:firmware>
+            ${CMAKE_BINARY_DIR}/firmware.hex
+    COMMAND arm-none-eabi-size $<TARGET_FILE:firmware>
+    VERBATIM
+)