Repeating background layer converts to a single intrinsic-size core/image (tiling lost) #3552

Problem

An empty element whose only output is a repeating background image is converted to a single core/image at the image's intrinsic size. A tiled texture that covers a whole section renders as one tile in the corner.

HtmlCompilation::backgroundImageBlockFromElement() reads only background-size. It ignores background-repeat (and background-position), although CoverStyleResolver::hasRepeatingBackground() already detects repetition (CoverPattern uses it to decline).

Reduced reproduction (released SSI 1.33.156)

tile.png is a 40×40 checker.

<style>body{margin:0}.band{position:relative;padding:80px 24px}.t{position:absolute;inset:0}h2{position:relative}</style>
<section class="band"><div class="t" style="background-image:url(tile.png);background-repeat:repeat"></div><h2>Answer every call</h2></section>

The quality gate reports 0 fallbacks, so the loss is invisible to the gate.

Expected

A repeating background stays a background. One option is a native container carrying style.background (backgroundImage, backgroundRepeat, backgroundSize, backgroundPosition), which core supports on core/group; this keeps the element's geometry, so the tiled texture fills the same box. Non-repeating cover/contain layers can keep today's image path.

Acceptance

• A regression test using the reduced page. It fails on trunk and passes with the fix: the repeating layer is not a core/image, and its block carries the repeat.
• Before/after render evidence: the painted box and repeat match the source.
• Existing suites pass.

This also blocks #3551 (#2613 class 1). That PR routes empty background-only custom elements (Wix wix-bg-image tiled overlays) through this same path. It removes the fallback, but on https://www.getringwave.com/ the tiled overlays would render as single tiles.

AI disclosure: Claude (Opus) via Roadie/Pi found this while reviewing #3551 on a real import, and measured it with Playwright, under Chris Huber's direction.