CI: build on bread-ecosystem's shared Arch container

Ports breadbox onto the shared, pinned CI build image (following the
pattern proven in breadpad): ci/build.sh delegates to bread-ecosystem's
own ci/build.sh, pinned by commit sha in ci/bread-ecosystem.rev, instead
of building directly on the bare self-hosted runner. Also adds
check.yml to run clippy/test on feature/fix branches ahead of the
dev-track release build.

No ci/deps.txt: breadbox's Cargo.toml pulls in nothing beyond what the
shared Containerfile already installs (gtk4, gtk4-layer-shell). The
librsvg system dep in bakery.toml is a runtime gdk-pixbuf loader for
SVG icons, not a Cargo build dependency.

ci/bread-ecosystem.rev

@@ -0,0 +1 @@
+147cfbbf96ae4b171027defa1130d2caddb934b1

ci/build.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+# Delegates to bread-ecosystem's shared CI build image/script, pinned to
+# the commit in ci/bread-ecosystem.rev — not `main`. bread-ecosystem's CI
+# files now affect every product's release pipeline, so bumping the pin
+# is a deliberate act instead of silent drift (see the bread-theme test
+# that broke here for exactly that reason, before it was pinned by rev).
+#
+# Usage: ci/build.sh cargo build --release --locked
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+REV="$(cat "${ROOT}/ci/bread-ecosystem.rev")"
+
+CACHE_DIR="/tmp/bread-ecosystem-ci-${REV}"
+if [ ! -d "$CACHE_DIR" ]; then
+    rm -rf /tmp/bread-ecosystem-ci-*
+    git clone https://git.breadway.dev/Breadway/bread-ecosystem.git "$CACHE_DIR"
+    git -C "$CACHE_DIR" checkout --quiet "$REV"
+fi
+
+bash "${CACHE_DIR}/ci/build.sh" breadbox "$ROOT" "$@"