[a11y][Breadcrumbs] Assess WCAG 2.2 conformance #49434

Context

Wave 4, item 5. Tracking issue: #49260.

Focus on the nav landmark and its label (1.3.1), aria-current on the last item (4.1.2), separators hidden from assistive technology, and the collapse button (expandText, 2.1.1).

Scope

1. Add axe tests and fixtures in test/regressions/a11y/fixtures/.
2. Write Breadcrumbs/accessibility.md.
3. Enroll it in the CSS-dependent suite (1.4.10, 1.4.4, 1.4.12).
4. Run pnpm a11y:scorecard.

Manual tests

Write each check in the report as numbered Manual testing steps, as in Button/accessibility.md. pnpm a11y:scorecard then adds them to manual-testing.md. For screen-reader steps, use NVDA with Chrome and VoiceOver with Safari.

• 1.3.1 — With a screen reader, list the landmarks. Breadcrumbs shows as a navigation landmark with the name "breadcrumb". The screen reader announces it as a list with the item count.
• 4.1.2 — The last item announces "current page" when the author sets aria-current="page".
• 1.3.1 — The separators are silent.
• 2.1.1, 2.4.3 — With collapsed items, press Tab to the ellipsis button. It announces its expandText name. Press Enter. The items expand, and focus moves to the first new item.
• 1.4.1, 1.4.3 — The links stay distinguishable from the current page without color. The link text is at least 4.5:1.
• 1.4.10 — At 320 CSS px width, the items wrap and no item is lost.

Done when

• Breadcrumbs has a report and is in the scorecard.
• Each manual test in this issue has steps and a result in the report.

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