Syncs with the stack now open upstream as pull requests: the progress bar
extracted into a shared command, \subsectionpage, \appendix, the dark
title band, and the metropolis-style dividers.
Two local changes are reapplied on top, as before: the licence notice in
the header, and font selection left to the document so this template can
use OpenSans and RobotoMono.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>