build/ is generated by CMake and must not be in the repo; it bloated .git to
571MB and made pushes fail (gitea/nginx resets the connection on large uploads).
Untrack the 397 build/ files (kept on disk, just removed from the index) and add
a .gitignore covering build/ and build_*/ so they stay out going forward.
Repo is now source-code only and pushes are small.
