feat(next): update bootc to v1.16.1 matching main branch

Fixes composefs splitstream format mismatch that prevented bootc
upgrade on :next. composefs-rs added repr(C) to SplitstreamHeader
between v1.15.x and v1.16.x, moving the magic bytes from offset 18
to offset 0. Users on :next with bootc 1.15.2 could not upgrade to
images built with the new format.

Syncs :next to the same bootc version as :main.

Closes projectbluefin/dakota issue 981

Assisted-by: Claude Sonnet 4.6 via GitHub Copilot

Author: castrojo