Issue #3592934: Add sticky header functional coverage

Functional coverage for the default theme sticky header (sticky_header setting, https://www.drupal.org/project/vartheme_bs5/issues/3593396).

Written against the default theme so it works whatever the default theme is, including a custom subtheme. All selectors are theme agnostic — HTML5 landmarks, ARIA roles, and Drupal core or module classes — so swapping the default theme does not break the tests.

Scenarios — tests/features/01-website-base-requirements/01-07-default-theme-settings.feature

• The default theme renders its main landmarks on the homepage.
• The sticky header is enabled by default on the homepage.
• The header gains the scrolled state when the page is scrolled.
• The sticky header stays sticky across mobile, tablet and desktop breakpoints.
• The sticky header setting is available in the default theme settings.
• Disabling the setting removes the sticky header; re-enabling restores it (verified as an anonymous visitor after flushing caches).

Performance budgets — tests/features/15-quality/15-03-performance.feature

• Tightened page load budgets to 3 seconds for the login page and 5 seconds for the other key pages, covering load time with the sticky header library (CSS + JS) active.

Supporting changes

• tests/selectors/default-theme.json — a theme-agnostic named selector registry, auto-loaded via worldParameters.selectors.
• tests/step-definitions/varbase.steps.js — steps to open the default theme settings, toggle the sticky header setting, flush all caches from the Drupal core Performance page, and assert an element is sticky.

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AI-Generated: Yes (Used Claude Code to draft the scenarios and steps; reviewed by the contributor.)

Checkpoints

• File an issue about this project
• Addition/Change/Update/Fix to this project
• Testing to ensure no regression
• Automated unit/functional testing coverage
• Developer Documentation support on feature change/addition
• User Guide Documentation support on feature change/addition
• UX/UI designer responsibilities
• Accessibility and Readability
• Code review from 1 Varbase core team member
• Full testing and approval
• Credit contributors
• Review with the product owner
• Update Release Notes
• Release