Ports breadpaper onto bread-ecosystem's shared, pinned Arch build
image/script instead of installing toolchain/libraries directly on the
bare self-hosted runner. Follows the pattern proven in breadpad:
ci/build.sh clones bread-ecosystem pinned to the sha in
ci/bread-ecosystem.rev and delegates to its ci/build.sh.
breadpaper needs nothing beyond the shared image's base package set
(only clap/anyhow as deps, no image-processing or GTK crates), so no
ci/deps.txt is added.
Also adds check.yml: clippy + test on feature/**/fix/** pushes, as a
fast pre-release-build signal, matching breadpaper's own build/test
flags (--release --locked, no --workspace since this isn't a
workspace).
