[a11y][BottomNavigation] Assess WCAG 2.2 conformance #49438

Context

Wave 4, item 9. Tracking issue: #49260.

Include BottomNavigationAction. Focus on actions without a visible label when showLabels is false (1.1.1, 2.5.3), the selected state (4.1.2, 1.4.1), the landmark (1.3.1) and 2.4.11 Focus Not Obscured (fixed position over content).

Scope

1. Add axe tests and fixtures in test/regressions/a11y/fixtures/.
2. Write BottomNavigation/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.

• 1.1.1, 2.5.3 — With showLabels={false}, focus each action with a screen reader. Each one announces its label, also when the label is not visible.
• 4.1.2 — Record how the screen reader announces the selected action. The author can add aria-current.
• 1.4.1 — In grayscale, the selected action stays distinguishable, for example by the visible label.
• 2.1.1 — Press Tab to each action and activate it with Enter and Space.
• 2.4.11 — With a fixed BottomNavigation, press Tab through the page content. It does not fully hide a focused element.
• 2.5.8 — Each action is at least 24 × 24 CSS px.

Done when

• BottomNavigation 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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