[frontend-contrib S4] P4 proof: SSR failure-containment fixtures

[rickylabs]

Part of #922

Scope

Fixtures proving the v1 containment contract: data-phase resolver catch -> quarantine card; client boundary post-hydration; route-level onError; documented page failure on SSR render throw (no error-boundary mode in Fresh's renderer).

Design source: RFC #890 (rfc.md §5) + .llm/runs/plan-frontend-contrib--seed/design/canonical/ (rev 3).

Acceptance

• gate: sync + async zone-throw fixtures assert the documented behavior
• gate: containment contract text matches observed behavior