Adopt Testing Library queries for interactive/accessible-element assertions

Why this is a different fix than #133

#133 states the rule ("assert on the rendered effect, not the prop passed into a mock"), but a written rule still depends on someone applying it — and the API itself works against that: item.props('active') is one call away, while asserting on real aria-current/aria-pressed state on the actual rendered element takes more effort with @vue/test-utils alone. Testing Library's design principle is the opposite: getByRole('link', { name: 'Documents', current: 'page' }) structurally cannot pass unless the real DOM has the real attribute — there's no props-only shortcut available.

Concretely, this would have caught the #130 sidebar-dedupe bug (two nav entries both getting aria-current="page"): a query like getByRole('link', { current: 'page' }) returning more than one element is a direct, obvious assertion failure, whereas checking items.map(i => i.props('active')) never touches the actual collision.

Proposed scope

• Add @testing-library/vue (verified: 8.1.0, vue: >= 3 peer dep, published 2026-08-07) and @testing-library/jest-dom (verified: 7.0.1, published 2026-08-09) alongside the existing @vue/test-utils suite — not a replacement. @vue/test-utils's shallowMount/prop-based assertions remain fine for pure logic/wiring checks where DOM fidelity isn't the point.
• Use Testing Library queries specifically for assertions on interactive elements and accessible/ARIA state (links, buttons, aria-current, aria-pressed, accessible names) — the exact category of thing #133's rule targets.
• One real worked example first, same approach as the composables adoption in #134: retrofit the #130 nav-dedupe fix (once merged) with a Testing-Library-style regression test, proving the pattern catches what the props-only assertion missed, before it spreads elsewhere.

Related

• #133 — states the rule this tooling enforces structurally rather than by convention.
• #134 — same "introduce one pattern via one real example, then let it spread opportunistically" shape.

Found during review of #125/#127/#130, all three of which hit this same gap independently.