HeroSimple emitted 'background' together with backgroundImage/Size/Position
that were undefined. React assigns '' for those on first render and Chromium
and Firefox then wipe the shorthand's gradient (background-image: none), so
the canvas hero rendered white. Build the style with only the relevant keys
(shared by render and toHtml); exported HTML is byte-identical.
Adds a real-browser (Chromium+Firefox) regression test: npm run test:browser:bg,
and toHtml byte-identity assertions.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
