Fix revealjs screen reader fragment announcements

[jooyoungseo]

Fixes #15019.

Description

Backport hakimel/reveal.js#3946 to the bundled reveal.js 5.1.0 without upgrading to 6.x.

Hidden incremental content is excluded from .aria-status until it is revealed, including nested fragments and pauses. The fragment itself can still be announced when activated, and the built-in initially visible and transient styles retain their behavior.

URL handling no longer navigates back to the current slide or fragment after writing its own history entry. Named horizontal slide indices are normalized, explicit fragment links still work, and browser Back can return to the state before the first fragment when fragment-in-url is enabled.

The readable source patch is retained in package/src/common/patches/ and reapplied by the HTML dependency updater before rebuilding both JavaScript bundles and source maps. Remove this backport when updating to an upstream release containing the fix.

Validation

• Added four Playwright scenarios covering actual live-region mutation counts, nested fragments, pauses, fragment URLs, and browser Back/Forward.
• 12/12 checks pass across Chromium, Firefox, and WebKit. The original three scenarios all fail on the unpatched bundle because the initial slide is announced twice.
• Rebuilt the UMD and ESM bundles from 5.1.0. The source maps contain source changes only in js/reveal.js and js/controllers/location.js.
• Local fixture rendering used Quarto 1.8.27, with the rebuilt bundles and current support plugin copied into the output before testing. The full Quarto build and test suite were not run.

Checklist

I have (if applicable):

• referenced the GitHub issue this PR closes
• updated the appropriate changelog in the PR
• ensured the present test suite passes
• added new tests
• created a separate documentation PR in Quarto's website repo and linked it to this PR

No format option or documentation change is required.

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