Derive BREAD_BINS from iso/bread-lockfile.toml and fail the bake when a
listed binary or breadhelp content is missing. Bake bakery share files
and drop breadcast from the copied installed.json. Add WebKitGTK 4.1 for
Tauri bos-settings, remove rustup, and rewrite README/DESIGN to match the
ISO+skel tree.
- .gitignore: ignore the /out/ ISO build dir
- bos-settings: use expect() with messages over unwrap() for piped stdio;
drop a stray blank line
- pacman.conf: reword the SigLevel=Never TODO as a future-improvement note