chore(opacity-checkerboard): add vrt coverage

[cdransf]

Description

Adds dedicated test/vrt/ coverage for the opacity-checkerboard shared CSS utility, following the button VRT pattern: opacity-checkerboard.vrt.ts covers size (m/s) across light/dark themes (the alternating square resolves one color via light-dark(), so dark theme is a real visual axis, not just a direction check), plus representative transparent-content fills, and a ForcedColors story covers the forced-color-adjust: none override; opacity-checkerboard-custom-properties.vrt.ts covers reference/override rows for the two exposed custom properties (--swc-opacity-checkerboard-square-dark-light/-dark-dark), each rendered in the color-scheme where it actually applies. No global-styles VRT file is included since this utility has no plain-class global stylesheet to cover. Also extracts the internal demo host (demo-opacity-checkerboard-swatch) and its swatch() helper out of the docs .stories.ts file into a shared module so both the docs and VRT stories use one implementation instead of two copies.

Motivation and context

Closes the VRT coverage gap for opacity-checkerboard called out in SWC-2422.

Related issue(s)

• Jira: SWC-2422

Author's checklist

• I have read the CONTRIBUTING and PULL_REQUESTS documents.
• I have reviewed the Accessibility Practices for this feature.
• I have added automated tests to cover my changes.
• I have included a well-written changeset if my change needs to be published.
• I have included updated documentation if my change required it.

Accessibility testing checklist

• Keyboard: N/A. Visual-only test coverage; no focusable parts, no interactive changes. Open Storybook → Opacity checkerboard → Opacity checkerboard VRT, press Tab through each story, and expect no element to receive focus since the pattern is decorative only.
• Screen reader: No accessibility-relevant changes. The demo host's existing role="img" + aria-label on the swatch, and aria-hidden="true" on the checkerboard layer, are untouched. Open Storybook → Opacity checkerboard → Opacity checkerboard VRT with VoiceOver/NVDA active, navigate to a swatch, and expect it to announce as an image with its label while the checkerboard layer itself is never announced.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 77f56ec

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[github-actions]

📚 Branch Preview Links

• Storybook (Gen2, dev mode)
• Storybook (Gen2, prod mode)
• Documentation Site (Gen1)
• Storybook (Gen1)

🔍 Gen1 Visual Regression Test Results

When a visual regression test fails (or has previously failed while working on this branch), its results can be found in the following URLs:

• Spectrum | Light | Medium | LTR
• Spectrum | Dark | Large | RTL
• Express | Light | Medium | LTR
• Express | Dark | Large | RTL
• Spectrum-two | Light | Medium | LTR
• Spectrum-two | Dark | Large | RTL
• High Contrast Mode | Medium | LTR

Deployed to Azure Blob Storage: pr-6619

If the changes are expected, update the current_golden_images_cache hash in the circleci config to accept the new images. Instructions are included in that file.
If the changes are unexpected, you can investigate the cause of the differences and update the code accordingly.

[pfulton]

Seems good to me! I left a comment about the suggested change to the demo component, but I'm fine with whatever y'all decide to do on that.

[cdransf]

Seems good to me! I left a comment about the suggested change to the demo component, but I'm fine with whatever y'all decide to do on that.

All set and reverted. ✨