feat: introduce selectors in the payload of preflight for highlighting

[dzehnder]

Description

Surface precise DOM locations for every preflight issue so the UI can highlight the offending elements directly. Adds a shared selector utility and threads those selectors through each audit response plus fixtures/tests.

Summary

• Introduced getDomElementSelector()/toElementTargets() helper for consistent minimal CSS selectors.
• Added element selector payloads to all preflight checks which are visible (DOM/body-size/lorem/H1, links, headings, readability, accessibility) so each opportunity points to its exact node.
• Updated affected fixtures and expectations to include the new selector fields.

[github-actions]

This PR will trigger a minor release when merged.

[codecov]

Codecov Report

❌ Patch coverage is 98.44358% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/metatags/seo-checks.js	78.94%	4 Missing ⚠️

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