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

justfile

@@ -0,0 +1,52 @@
+# ── variables ──────────────────────────────────────────────────────────────
+
+image     := "ptt-builder"
+build_dir := "build"
+
+# podman takes priority over docker
+engine := `command -v podman 2>/dev/null \
+           || command -v docker 2>/dev/null \
+           || { echo "error: podman or docker required" >&2; exit 1; }`
+
+# user-namespace mapping: podman rootless vs docker
+user_ns := `command -v podman >/dev/null 2>&1 \
+            && echo "--userns=keep-id" \
+            || echo "--user $(id -u):$(id -g)"`
+
+# host CPU arch → container build arg
+arch := `uname -m | sed 's/x86_64/amd64/; s/aarch64/arm64/'`
+
+# ── recipes ────────────────────────────────────────────────────────────────
+
+# build container image (only rebuilt when Dockerfile changes)
+image-build:
+    {{engine}} build \
+        --build-arg TARGETARCH={{arch}} \
+        --build-arg GCC_VERSION=13.2.rel1 \
+        -t {{image}} .
+
+# compile firmware inside the container
+build: image-build
+    {{engine}} run --rm \
+        {{user_ns}} \
+        -v "{{justfile_directory()}}:/src:z" \
+        {{image}} \
+        sh -c "cmake -B /src/{{build_dir}} -G Ninja \
+                     -DCMAKE_BUILD_TYPE=MinSizeRel /src \
+               && ninja -C /src/{{build_dir}}"
+
+# flash firmware via pyocd on the host (requires USB / DAPLink)
+flash: build
+    pyocd flash --target nrf52840 {{build_dir}}/firmware.hex
+
+# start GDB server for debugging
+gdbserver:
+    pyocd gdbserver --target nrf52840 --port 3333
+
+# remove build artifacts
+clean:
+    rm -rf {{build_dir}}
+
+# remove build artifacts AND the container image
+clean-all: clean
+    {{engine}} rmi {{image}} 2>/dev/null || true