Accessibility regression gate

The accessibility quality gate is now active in CI and blocks merges on serious or critical violations from the automated scans. During rollout we tuned selectors to avoid false positives from test fixtures and confirmed that flaky checks were tied to unstable navigation timing, now fixed with deterministic waits. Initial impact shows longer pipeline time by about ninety seconds, but this is offset by fewer post-merge remediations and clearer ownership in PR review. Next phase is adding a small manual checklist for keyboard flow and focus order in pages with dynamic filter behavior.