Attach button inner-fill combinators to every selector

[chubes4]

Closes #1306.

Problem

#1304 appends > .wp-block-button to the wrapper prelude. When that prelude lists desktop and mobile markers, CSS applies the combinator only to the last selector:

:where(.control-163):where(.wp-block-buttons),
:where(.control-172):where(.wp-block-buttons)> .wp-block-button {
  width: 100% !important;
}

The first selector is .wp-block-buttons { width: 100% }. After #1304, Nimbus Contacts wrapper is 1440px and the inner link is still 7×88.

Change

Split the prelude and append the combinator to each selector.

Coverage

tests/unit/button-wrapper-fill-selector-list.php fails on trunk (combinator only on the last marker) and passes here. composer test exits 0.

AI assistance

Claude Sonnet 4.5 via OpenCode measured the v8 import after #1304 and implemented the split. Chris Huber reviewed.

[chubes4]

The solved-site-promotion gate initially failed here: 15-saas regressed by 6/9955320 pixels.

Cause was #1304's box-sizing:border-box on the inner control, not the selector-list split. 15-saas has .tier-cta { width: 100% } on .btn-outline controls carrying border: 1px solid; switching those to a border box moved their edges.

Fixed in d429aa1 by emitting width only. Gate is green.

AI assistance: Claude Sonnet 4.5 via OpenCode read the gate evidence, traced the fixture CSS, and made the change. Chris Huber reviewed.