Add automated accessibility regression tests for the front page contrast and heading

Problem/Motivation

Two accessibility defects were found on a sibling site template by an axe-core scan and fixed today:

• Footer social icons failing WCAG 2.1 AA contrast at 4.27:1 against the required 4.5:1 (axe rule color-contrast, impact serious). Fixed in #3621197.
• A front page with no top-level heading (axe rule page-has-heading-one, impact moderate). Addressed by #3621201 and #3621200.

The quality suite here already runs axe-core through varbase-e2e, but it gates on impact level rather than on the specific rules, so a regression in either of these would not be reported by name.

Note that this template already places a heading at level: 1 on every Canvas page, including the home page, so it is not expected to be failing page-has-heading-one today. The scenario is added as a regression guard, not as a fix.

Steps to reproduce

1. Read the accessibility feature file in the quality suite.
2. There is no scenario naming color-contrast or page-has-heading-one.

Proposed resolution

Add rule-specific scenarios to the existing accessibility feature, in the file's own style and tags, using the step form it already uses:

Then the page should not violate the accessibility rule "color-contrast"
Then the page should not violate the accessibility rule "page-has-heading-one"

A rule-specific scenario only has value if the element under test is actually rendered in CI. Confirm the footer social menu links exist on the CI-installed site before relying on the contrast scenario; if they do not, say so and cover it another way rather than adding a scenario that passes vacuously.

If the contrast scenario fails here, that is a real finding: this template's theme is a clone of Vartheme BS5 and may carry the same footer social icon colour. Report it rather than relaxing the test.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ✅ Testing to ensure no regression
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ✅ Release, horizonaid-1.0.0

User interface changes

• None. Test coverage only.

API changes

• None.

Data model changes

• None.

Release notes snippet

• Add automated accessibility regression tests for the front page colour contrast and top-level heading.