[a11y] Focus guard sentinels are exposed as nameless buttons in WebKit (axe button-name violation)

### Summary

The visually-hidden focus-guard sentinels Base UI renders around trapped popups —

```html

```

— are focusable (`tabindex="0"`), carry `role="button"`, and have **no accessible name**. In Chromium and Firefox they are excluded from the accessibility tree, so axe passes. In **WebKit** they are exposed, and axe flags **`button-name`** (serious) on every guard.

### Environment

- `@base-ui/react` **1.6.0**
- Playwright **1.61.1** (`webkit`), `axe-core` **4.12.1** via `@axe-core/playwright`
- Reproduces deterministically (not load/timing dependent), light theme, headless

### Repro

1. Render any focus-trapping surface (observed with a Dialog-based sheet/drawer and a command-palette dialog) in the open state.
2. Run an axe scan in Playwright WebKit.
3. `button-name` violations are reported against the `[data-base-ui-focus-guard]` spans (selector observed: `span[data-type="inside"][data-base-ui-focus-guard=""][role="button"]`). The same scan passes in Chromium and Firefox.

### Expected

Focus guards should be invisible to assistive technology — e.g. `aria-hidden="true"` on the sentinels (they are focus plumbing, not actionable controls), or an accessible name if they are intentionally exposed.

### Impact

- Safari/VoiceOver users can land on an unnamed "button" at popup boundaries.
- Cross-engine axe gates (CI a11y suites running the WebKit engine) fail on every story/page with an open trapped popup, with no workaround available from userland — the guards are re-rendered by Base UI on focus changes, so consumer-side DOM patching is not durable.

Happy to provide a minimal repro repository if useful.

Start by reproducing the open trapped-popup case in Playwright WebKit and inspect span[data-type="inside"][data-base-ui-focus-guard=""][role="button"]. Trace where these focus guards are rendered and rerendered; done means WebKit axe scans report no button-name violations while popup focus trapping still works.

Written by the indexing model from the issue text.