Free GPIO on the 14-pin header for external hardware sync?

[KanjiKaneko]

Hi, and thanks for this great platform.

For a research setup, I need to time-synchronize an external
microcontroller with OpenEarable 2 using a shared hardware sync line
(timestamping signal edges on both sides).

1. Is there a GPIO on the 14-pin header that is free to use from the
   application firmware, i.e. not already assigned to another peripheral?
   From the block diagram the header's GPIO net seems to also route to the
   flexible sensor PCB, so I wasn't sure if it's already used or available.

2. If so, which nRF5340 pin (P0.xx / P1.xx) does it map to? I'd like to
   confirm it can be used with GPIOTE + (D)PPI for hardware timestamping.

3. Could you point me to the device-tree / pinctrl file (under boards/ or
   dts/) where the header pin assignments are defined?

Thanks!