Update @vardot/varbase-e2e to 2.0.7 and add accessibility structure coverage

Problem/Motivation

@vardot/varbase-e2e 2.0.7 turns the impact gate into a ladder: no serious accessibility violations now fails on critical as well. It also adds a one-step full check, element-scoped rule assertions, and probes that skip what assistive technology cannot reach.

Varbase Starter is pinned to ^2.0.6, so none of that applies here yet.

15-02-accessibility.feature gates on critical only, with a note saying to tighten to serious or a per-page AA audit once the theme is clean at that level. It is worth checking whether that is now true.

Proposed resolution

• Bump @vardot/varbase-e2e to ^2.0.7.
• Add 15-04-accessibility-structure.feature: document structure and identifiable controls across the six shipped front-end pages.
• Tighten 15-02 to the serious gate and a full AA audit on the pages that are clean at that level today.
• Refresh the 15-quality README counts.

Measured on a fresh install, the structure and control checks pass on all six pages. The serious gate and the AA audit pass on the home page and a blog article, and fail on /about-varbase, /features, /blog and /contact-us against a single rule: color-contrast, 2 nodes.

That rule is the stock Bootstrap palette this starter kit ships rather than a markup defect, so those four pages keep the critical gate and the file says why. They can be moved up when the palette is settled. This issue does not change any colour.

Verified: the 15-quality bucket is 42 scenarios, 42 passed.

Remaining tasks

✅ Update to 2.0.7
✅ Add the accessibility structure coverage
✅ Tighten the gate where the theme is clean
✅ Prove the suite green on a fresh install
❌ Remove the temporary twig/twig:<3.30 pin from .gitlab-ci.yml once a Twig release compatible with Drupal core is out. It pins the CI build only and does not constrain the recipe.
➖ Review
➖ Release

User interface changes

None. Test-only.

API changes

None.

Data model changes

None.