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

link/nrf52840.ld

@@ -0,0 +1,66 @@
+/*
+ * nRF52840  -  no SoftDevice
+ * Flash: 1 MB @ 0x00000000
+ * RAM:   256 KB @ 0x20000000
+ *
+ * Before first flash, erase the entire chip to remove any SoftDevice:
+ *   pyocd erase --target nrf52840 --chip
+ */
+
+MEMORY
+{
+    FLASH (rx)  : ORIGIN = 0x00000000, LENGTH = 1024K
+    RAM   (rwx) : ORIGIN = 0x20000000, LENGTH = 256K
+}
+
+/* stack grows downward from the top of RAM */
+_estack = ORIGIN(RAM) + LENGTH(RAM);
+
+ENTRY(Reset_Handler)
+
+SECTIONS
+{
+    /* vector table must be at the start of Flash */
+    .isr_vector :
+    {
+        KEEP(*(.isr_vector))
+        . = ALIGN(4);
+    } > FLASH
+
+    /* code and read-only data */
+    .text :
+    {
+        *(.text .text.*)
+        *(.rodata .rodata.*)
+        . = ALIGN(4);
+        _etext = .;
+    } > FLASH
+
+    /* load address of .data in Flash  -  Reset_Handler copies from here */
+    _sidata = LOADADDR(.data);
+
+    /* initialized variables: stored in Flash, run from RAM */
+    .data :
+    {
+        _sdata = .;
+        *(.data .data.*)
+        . = ALIGN(4);
+        _edata = .;
+    } > RAM AT > FLASH
+
+    /* zero-initialized variables: Reset_Handler clears this region */
+    .bss (NOLOAD) :
+    {
+        _sbss = .;
+        *(.bss .bss.*)
+        *(COMMON)
+        . = ALIGN(4);
+        _ebss = .;
+    } > RAM
+
+    /DISCARD/ :
+    {
+        *(.ARM.exidx*)
+        *(.gnu.linkonce.armexidx.*)
+    }
+}