1613: Section Color: render One column through the shared layout organism

[dblanken-yale]

1613: Section Color: Audit block contrast against the new One column / 70-30 section backgrounds

Description of work

• Other work completed in: 1613: Section Color: Audit block contrast against the new One column / 70-30 section backgrounds yalesites-project#1514, 1613: Section Color: Audit block contrast against the new One column / 70-30 section backgrounds component-library-twig#707

Based on 1616-section-color-parity, not develop.

This branch exists for two reasons, and the first one is the fix for the blocking finding on
yalesites-org/yalesites-project#1514.

Why the One column background did not render on the multidev

The review found that a One column section with a Component Theme rendered as a bare
<div class="layout layout--onecol"> on the PR multidev, while the 70/30 section on the same page
worked. Neither of the suspected causes was it: the theme value is saved through the editor
form, and it does reach the template.

yalesites_profile/composer.json pins yalesites-org/atomic: 1.82.0, and the One column theme
wiring exists only on the 1616-section-color-parity branch of this repo — the released tags do
not have it:

$ git show v1.83.0:templates/layouts/layout--onecol.html.twig | grep -c yds-layout
0

A Pantheon multidev only swaps in companion branches whose name matches the pushed branch, and
this repo had no 1613-block-contrast-section-backgrounds branch, so the multidev built against
released atomic 1.82.0, whose onecol template emits no section markup at all. That also explains
the part of the evidence that pointed away from the real cause: both CSS rules the review found
are present, because .layout--onecol.yds-layout ships from ys_layouts (on the PR branch) and
.yds-layout[data-component-theme] has been in this theme's dist for a long time. CSS present
plus template absent is exactly the released-atomic combination.

So: this branch existing is the fix. It is an integration gap, not a code bug.

The change itself

layout--onecol.html.twig now embeds @organisms/layout/layout with
component__layout: 'one-column' rather than adopting the yds-layout class and
data-component-theme attribute by hand, so all four Layout Builder sections emit one signature
from one template:

<div class="yds-layout layout layout--onecol" data-component-theme="one"
     data-component-layout="one-column" data-component-padding="default"
     data-component-has-divider="false" data-component-width="site">

layout--onecol is passed through as an extra class because core's stylesheets and both of the
page's margin rules (.main-content > *:first-child in page-layouts.scss,
.main-content .layout.layout--onecol:last-of-type in this theme's layout-builder.css) select on
it.

What changes for an unthemed section. It now carries data-component-theme="default" where it
previously carried no section markup at all. That was the main risk and it was checked rather than
assumed: everything --color-layout-border feeds — the CTA atom's fill, the divider atom, tabs,
pull-quote — is already guarded on
[data-component-theme]:not([data-component-theme='default']), so an unthemed section picks up
none of it. Two supporting changes live in the companion PRs: _yds-layout.scss excludes
one-column from the default-theme section-margin rule, and onecol.css scopes its vertical
padding to a themed section now that yds-layout is no longer a themed-only signal.

The "render nothing for an unthemed empty section" guard is deliberately unchanged. The post,
event and page default displays each ship a trailing empty one-column Content Section, so rendering
those would hand :last-of-type to an empty wrapper and drop the pre-footer margin on every node
using them.

Functional testing steps:

1. Check out 1613-block-contrast-section-backgrounds in this repo, component-library-twig and
   yalesites-project (all based on 1616-section-color-parity).
2. npm run build in component-library-twig, then lando drush cr.
3. lando drush php:script scripts/local/1613-section-contrast-fixture.php
4. Visit /1613-one-column-section-backgrounds and confirm all six backgrounds paint, and that
   each section's markup carries both data-component-theme and
   data-component-layout="one-column".
5. Visit any ordinary page and confirm an unthemed One column section is visually unchanged and now
   reports data-component-theme="default".
6. Test the editor path specifically, per the review: add a One column section through Layout
   Builder, set a Component Theme, save, and confirm it paints.

Verification performed

• One column measured in Drupal across all six section themes; every one paints, with a correctly
  inverted foreground: one rgb(0,54,107), two rgb(247,247,247), three rgb(33,33,33), four
  rgb(39,107,190), five rgb(217,217,217), six rgb(97,168,255).
• Unthemed pages confirmed rendering data-component-theme="default" data-component-layout="one-column".
• Storybook confirms single-region rendering: yds-layout layout, one-column, no __inner, no
  __primary, no __secondary, 0 dividers.
• Screenshots (Drupal and Storybook) are committed in
  1613: Section Color: Audit block contrast against the new One column / 70-30 section backgrounds component-library-twig#707 under
  components/03-organisms/layout/layout/screenshots/.

References yalesites-org/YaleSites-Internal#1613