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

include/fhss.h

@@ -0,0 +1,6 @@
+#pragma once
+#include <stdint.h>
+
+void    fhss_init(void);
+uint8_t fhss_next_channel(void);   /* next channel from AES-ECB PRNG sequence */
+void    fhss_sync_tick(void);      /* call every FHSS_DWELL_MS ms */

include/power.h

@@ -0,0 +1,4 @@
+#pragma once
+
+void power_init(void);
+void power_sleep_until_button(void);   /* SYSTEM_ON WFI, woken by GPIOTE event */

include/radio.h

@@ -0,0 +1,7 @@
+#pragma once
+#include <stdint.h>
+
+void    radio_init(void);
+void    radio_set_channel(uint8_t ch);        /* 0–39, maps to 2402–2480 MHz */
+void    radio_tx(const uint8_t *data, uint8_t len);
+void    radio_tx_burst(void);                 /* TX with FHSS hopping */

include/regs.h

@@ -0,0 +1,90 @@
+#pragma once
+#include <stdint.h>
+
+/*
+ * Hardware register bitfield unions for nRF52840 peripherals.
+ * Layout is guaranteed correct only with arm-none-eabi-gcc (LSB-first bitfields).
+ * Bit ranges match nRF52840 Product Specification v1.7.
+ */
+
+/* ── GPIOTE ──────────────────────────────────────────────────────────────── */
+
+/* CONFIG[n] — channel configuration */
+typedef union {
+    struct {
+        uint32_t MODE     : 2;   /* [1:0]   0=Disabled  1=Event  3=Task */
+        uint32_t          : 6;   /* [7:2]   reserved */
+        uint32_t PSEL     : 5;   /* [12:8]  pin number within port */
+        uint32_t PORT     : 1;   /* [13]    0=Port0  1=Port1 */
+        uint32_t          : 2;   /* [15:14] reserved */
+        uint32_t POLARITY : 2;   /* [17:16] 0=None  1=LoToHi  2=HiToLo  3=Toggle */
+        uint32_t          : 2;   /* [19:18] reserved */
+        uint32_t OUTINIT  : 1;   /* [20]    initial output value for Task mode */
+        uint32_t          : 11;  /* [31:21] reserved */
+    } bit;
+    uint32_t reg;
+} gpiote_config_t;
+
+#define GPIOTE_MODE_DISABLED  0u
+#define GPIOTE_MODE_EVENT     1u
+#define GPIOTE_MODE_TASK      3u
+#define GPIOTE_POL_NONE       0u
+#define GPIOTE_POL_LOTOHI     1u
+#define GPIOTE_POL_HITOLO     2u
+#define GPIOTE_POL_TOGGLE     3u
+
+/* INTENSET / INTENCLR — interrupt enable */
+typedef union {
+    struct {
+        uint32_t IN0  : 1;   /* [0]    channel 0 input event */
+        uint32_t IN1  : 1;   /* [1]    channel 1 input event */
+        uint32_t IN2  : 1;   /* [2] */
+        uint32_t IN3  : 1;   /* [3] */
+        uint32_t IN4  : 1;   /* [4] */
+        uint32_t IN5  : 1;   /* [5] */
+        uint32_t IN6  : 1;   /* [6] */
+        uint32_t IN7  : 1;   /* [7] */
+        uint32_t      : 23;  /* [30:8]  reserved */
+        uint32_t PORT : 1;   /* [31]    PORT event */
+    } bit;
+    uint32_t reg;
+} gpiote_inten_t;
+
+/* ── RADIO ───────────────────────────────────────────────────────────────── */
+
+/* FREQUENCY — radio channel */
+typedef union {
+    struct {
+        uint32_t FREQUENCY : 7;   /* [6:0]  offset from base frequency in MHz */
+        uint32_t           : 1;   /* [7]    reserved */
+        uint32_t MAP       : 1;   /* [8]    0: base=2400 MHz  1: base=2360 MHz */
+        uint32_t           : 23;  /* [31:9] reserved */
+    } bit;
+    uint32_t reg;
+} radio_frequency_t;
+
+#define RADIO_MAP_DEFAULT 0u   /* channel n → 2400+n MHz */
+#define RADIO_MAP_BLE     1u   /* channel n → 2360+n MHz */
+
+/* TXPOWER — transmit power */
+typedef union {
+    struct {
+        int32_t TXPOWER : 8;   /* [7:0]  signed dBm: +8, +7, +6, +5, +4, +3, +2,
+                                          0, -4, -8, -12, -16, -20, -40 */
+        uint32_t        : 24;  /* [31:8] reserved */
+    } bit;
+    uint32_t reg;
+} radio_txpower_t;
+
+/* MODE — radio data rate and modulation */
+typedef union {
+    struct {
+        uint32_t MODE : 4;   /* [3:0]  0=NRF_1Mbit  1=NRF_2Mbit  4=BLE_1Mbit … */
+        uint32_t      : 28;  /* [31:4] reserved */
+    } bit;
+    uint32_t reg;
+} radio_mode_t;
+
+#define RADIO_MODE_NRF_1MBIT  0u
+#define RADIO_MODE_NRF_2MBIT  1u
+#define RADIO_MODE_BLE_1MBIT  4u