feat(back-to-top): port pf-v5-back-to-top to pf-v6-back-to-top

[zeroedin]

Summary

• Removes <pf-v5-back-to-top>
• Ports <pf-v5-back-to-top> to <pf-v6-back-to-top> for PatternFly v6
• Shortcut to navigate to the top of a lengthy content page
• Renders as <a> with href (progressive enhancement) or <button> (JS scroll)
• Supports always-visible, scrollable-selector, accessible-label, and href attributes
• delegatesFocus for keyboard accessibility
• Respects prefers-reduced-motion for scroll behavior
• Demos: basic (index), always-visible, button-mode, scrollable-container

Closes #2980
Depends on #3130

Intentional divergences from React

• href attribute: renders as an <a> link instead of a <button>, enabling progressive enhancement (works without JS)
• Default slot replaces React's title prop, allowing rich content for the button text
• accessible-label abstracts aria-label per ADVICE.md
• always-visible attribute instead of React's isAlwaysVisible (no is- prefix per ADVICE.md)
• scrollable-selector dash-case attribute for the camelCase property
• prefers-reduced-motion handled via matchMedia check (instant vs smooth scroll)

Test plan

• npm run test passes
• Demos render correctly at localhost:8000 with ?rendering=chromeless
• Button appears after scrolling past threshold (~400px)
• always-visible keeps button visible without scroll
• href mode renders as link and navigates to fragment
• scrollable-selector spies on custom container scroll
• Screen reader announces accessible name
• prefers-reduced-motion: reduce uses instant scroll

Assisted-By: Claude noreply@anthropic.com

Testing Instructions

Notes to Reviewers

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🦋 Changeset detected

Latest commit: 059edbd

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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@patternfly/elements	Major

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More Info

{
  "valid": true,
  "errors": [],
  "warnings": [],
  "input": "feat(back-to-top): port pf-v5-back-to-top to pf-v6-back-to-top"
}

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