Add ssrFixture() and an opt-in a11y assertion to the test layer

[vivek7405]

Problem

The shipped fixture() renders via renderToString and parses HTML into a container; it never drives client hydration or awaits a true update cycle (waitForUpdate yields two macrotasks, never el.updateComplete), and there is no packaged ssrFixture() that server-renders then hydrates in one call. Separately, nothing touches accessibility testing: no axe-core integration, no assertion helper, and the scaffold browser test only checks the DOM exists. Both live in the existing real-Chromium WTR layer.

Design / approach

Export a small ssrFixture() that renders then hydrates in the existing WTR Chromium session awaiting the native el.updateComplete, plus an opt-in axe assertion. Thin wrappers over the real browser already running.

Web-standards fit: Runs in real Chromium and awaits the native updateComplete; the helpers are small and axe is opt-in, not a forced gate.

Prior art: Lit @lit-labs/testing ssrFixture/csrFixture + WTR SSR plugin; Lit @open-wc/testing + axe for a11y.

Acceptance criteria

• A packaged ssrFixture() server-renders then hydrates a component in the WTR Chromium layer and awaits el.updateComplete
• ssrFixture() is exported from the testing module alongside fixture() and documented in agent-docs/testing.md
• An opt-in assertNoA11yViolations(el) helper runs axe in the WTR Chromium session, not a forced gate
• The scaffold browser test demonstrates assertNoA11yViolations
• Tests prove ssrFixture catches a hydration-mismatch counterfactual and the a11y helper flags a missing-label/role case
• Tests cover the new behaviour at the applicable layer(s)
• Docs / AGENTS.md / CONVENTIONS.md updated if the public surface changed

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