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 +1,23 @@
+FROM debian:bookworm-slim
+
+ARG GCC_VERSION=13.2.rel1
+ARG TARGETARCH=amd64
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        wget xz-utils cmake ninja-build \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN case "${TARGETARCH}" in \
+        amd64) _arch=x86_64  ;; \
+        arm64) _arch=aarch64 ;; \
+        *) echo "unsupported arch: ${TARGETARCH}" >&2 && exit 1 ;; \
+    esac \
+    && wget -qO /tmp/gcc.tar.xz \
+        "https://developer.arm.com/-/media/Files/downloads/gnu/${GCC_VERSION}/binrel/arm-gnu-toolchain-${GCC_VERSION}-${_arch}-arm-none-eabi.tar.xz" \
+    && tar -xf /tmp/gcc.tar.xz -C /opt \
+    && ln -s "/opt/arm-gnu-toolchain-${GCC_VERSION}-${_arch}-arm-none-eabi" /opt/arm-toolchain \
+    && rm /tmp/gcc.tar.xz
+
+ENV PATH="/opt/arm-toolchain/bin:${PATH}"
+
+WORKDIR /src