ci: build against bread-ecosystem's shared Arch CI image, add check.yml

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.

ci/bread-ecosystem.rev

@@ -0,0 +1 @@
+620c5a1317a6b57276eabca961facdb78bf510db