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

src/power.c

@@ -0,0 +1,37 @@
+#include "power.h"
+#include "regs.h"
+#include <nrf52840.h>
+#include <cmsis_gcc.h>
+
+/* P0.02 on XIAO BLE — adjust to match your schematic */
+#define BUTTON_PIN  2u
+
+void power_init(void)
+{
+    /* DC/DC converter has lower quiescent current than the LDO */
+    NRF_POWER->DCDCEN = 1u;
+
+    NRF_GPIOTE->CONFIG[0] = (gpiote_config_t){
+        .bit = {
+            .MODE     = GPIOTE_MODE_EVENT,
+            .PSEL     = BUTTON_PIN,
+            .PORT     = 0u,
+            .POLARITY = GPIOTE_POL_LOTOHI,
+        }
+    }.reg;
+
+    NRF_GPIOTE->INTENSET = (gpiote_inten_t){ .bit.IN0 = 1u }.reg;
+
+    NVIC_EnableIRQ(GPIOTE_IRQn);
+}
+
+void power_sleep_until_button(void)
+{
+    NRF_POWER->TASKS_LOWPWR = 1u;
+    __WFI();
+}
+
+void GPIOTE_IRQHandler(void)
+{
+    NRF_GPIOTE->EVENTS_IN[0] = 0u;
+}