Fix screen reader fragment announcements and history navigation

[jooyoungseo]

Fixes #3945.

Changes

• Exclude hidden fragment descendants from slide and fragment announcements. Use fragment classes rather than computed visibility, which can be mid-transition. Preserve the built-in initially visible styles, current-only styles, custom fragments, and the announced fragment itself.
• Ignore hash changes that already describe the current slide and fragment. Normalize the missing vertical index on named horizontal slides, preserve explicit named fragment links when fragmentInURL is disabled, and restore the pre-fragment state when browser history returns to a fragment-less URL with the option enabled.
• Compute and announce fragment text after the entire fragment-class update loop. This fixes stale nested visibility when stepping backward and preserves ordered announcements for same-index groups.
• Add accessibility regression coverage for fragment visibility, backward navigation, grouped current-visible fragments, and URL navigation.

Validation

• Accessibility regression page: 25/25 assertions pass. The four new assertions fail before the deferred-announcement change, covering nested normal/fade-out backward navigation and same-index current-visible groups.
• Ran all 18 QUnit HTML pages sequentially: 444/445 assertions pass, including 214/214 on the main test page.
• One existing test-markdown.html assertion, External / Horizontal separator, fails with expected 2 vs actual 3 on both the patched and pristine upstream source (69/70 assertions pass). The initial parallel npm test run stalled locally and was stopped; this is not a claim that the full default command passed.
• Core build passed. Generated distribution files are not included in this source change.

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