test: cover configurable hero header section class

Add a quickstart phase that builds two single pages with featured images,
one overriding header_section_class via front matter and one relying on the
theme default, then asserts the override is applied, does not leak to other
pages, and the historical default spacing is preserved.

Refs #504

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>