Restore docs home page skip-link target and prevent mobile overflow from hidden labels

[Copilot]

The docs home page regressed in two user-visible ways on mobile: the rendered page no longer exposed the expected Starlight skip-link target, and hidden accessibility labels were still contributing to horizontal overflow on narrow phone viewports.

• Skip link / main landmark

  • Switched the custom skip link back to #starlight__main so it matches the Starlight page structure expected by downstream checks and assistive tooling.
  • Updated the client-side landmark enhancer to set id="starlight__main" on <main> and preserve programmatic focus with tabindex="-1".
  • Added a hidden #main-content alias inside <main> to keep older internal references working without changing the visible structure.

• Mobile overflow containment

  • Added a global .sr-only utility rule in docs CSS so visually hidden text is clipped correctly and cannot expand scroll width.
  • This specifically addresses long hidden anchor labels generated around section headings, which could still widen the page on 360–428px layouts.

• Regression coverage

  • Expanded the responsive Playwright matrix to include the affected narrow phone widths.
  • Added a focused regression check for the skip-link target on the home page.
  • Updated the overflow assertion to inspect the page scrolling element rather than only document.body.

Example of the restored skip-link contract:

<a href="#starlight__main" class="skip-link">Skip to main content</a>
<main id="starlight__main" tabindex="-1">
  <span id="main-content" class="sr-only" aria-hidden="true"></span>
  ...
</main>

[github-actions]

Thanks for the clear write-up and for calling out the mobile skip-link and overflow regressions specifically. This looks aligned and focused: the docs accessibility fix is paired with targeted Playwright regression coverage, and I did not see any new dependencies added.

The PR description is strong and the included tests make this easy to review. LGTM from a contribution-guidelines perspective.

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