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

Dockerfile

@@ -0,0 +1,11 @@
+FROM debian:bookworm-slim
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        gcc-arm-none-eabi \
+        binutils-arm-none-eabi \
+        libnewlib-arm-none-eabi \
+        cmake \
+        ninja-build \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /src