Same fix as breadpad: build inside the shared pinned Arch container
(bread-ecosystem/ci/, cloned at the sha in ci/bread-ecosystem.rev)
instead of building natively against whatever's on the runner host.
Adds check.yml (clippy + test on feature/**/fix/**) as a fast-fail gate
before anything reaches main.
Verified locally: build, clippy, and test all pass through the new
container path — no pre-existing lint/test debt found here.
