test(site): stabilize landing smoke contract

[StringKe]

Summary

• add a dedicated data-smoke-architecture contract to the Nimbus landing page
• make the three-worker smoke check query the stable contract instead of a presentation CSS class
• prevent future visual refactors from silently invalidating the smoke selector

Root cause

The redesigned landing page replaced .architecture-panel with .architecture-list, while scripts/run-three-worker-browser-smoke.mjs still waited for the removed class. The rendered page was healthy, but the main CI smoke timed out on the stale selector.

Validation

• PASS node --check scripts/run-three-worker-browser-smoke.mjs
• PASS pnpm run test:web-routes (19 tests)
• PASS pnpm run security:secret-scan
• PASS pnpm --filter @xid-kit/site check
• PASS pnpm --filter @xid-kit/site test (298 tests)
• PASS pnpm --filter @xid-kit/site build (360 pages, route audit PASS)
• PASS git diff --check
• SKIP local browser run by explicit request; the existing main CI performs the authoritative three-worker browser smoke after merge