[frontend-contrib S18] defineFrontendTestSuite + budgets enforcement

[rickylabs]

Part of #922

Scope

./testing entrypoint: generated host fixture running schema/resolution, island-props serialization round-trip, SSR, hydration, browser smoke, a11y (axe + keyboard), base-path, local+JSR modes; asserts the envelope's FrontendBudgets.

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

Acceptance

• gate: workers dogfood suite green in CI
• gate: budget violation fails the suite with the named budget