feat: add visual regression testing workflow

[apermo]

Summary

• Add reusable-wp-visual-regression.yml workflow for Playwright-based screenshot comparison testing against wp-env
• Supports configurable thresholds, viewports, color schemes, multisite, Mailpit, and snapshot regeneration
• Document the new workflow in README, CLAUDE.md inventory, and CHANGELOG

Test plan

• Verify YAML syntax is valid (no workflow_call parse errors)
• Confirm matrix-prep job correctly generates WP version and multisite matrices
• Test in a consuming repo with .wp-env.json and Playwright visual tests
• Verify update-snapshots: true produces downloadable snapshot artifacts
• Confirm failure case uploads visual diff artifacts

Closes #18

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request expands the project's testing capabilities by adding a reusable workflow designed to automate visual regression testing for WordPress projects. By leveraging Playwright, this addition allows developers to easily integrate screenshot comparison into their CI pipelines, complete with support for snapshot management and environment-specific configurations.

Highlights

• New Workflow Added: Introduced a reusable GitHub Actions workflow for Playwright-based visual regression testing in WordPress environments.
• Configuration Support: The new workflow supports customizable thresholds, viewports, color schemes, multisite configurations, and Mailpit integration.
• Documentation Updates: Updated README.md, CLAUDE.md, and CHANGELOG.md to include instructions and details for the new visual regression testing capability.

Ignored Files

• Ignored by pattern: .github/workflows/** (1)
  • .github/workflows/reusable-wp-visual-regression.yml

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Screenshots captured in the light, Comparing pixels, doing it right. With Playwright's power, tests now run, Visual regression, finally done.

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[gemini-code-assist]

Code Review

This pull request introduces documentation and changelog entries for a new Playwright-based visual regression testing workflow, reusable-wp-visual-regression.yml. However, the actual workflow file itself is missing from the current changes. Feedback focuses on lowering the default pixel difference threshold to improve testing accuracy and ensuring the documentation example pins the workflow version to a major version (e.g., @v0) instead of @main to align with the repository's style guide.

[gemini-code-assist]

The workflow file reusable-wp-visual-regression.yml is documented and added to the changelog, but the file itself is missing from the pull request. Please ensure the new workflow file is included in the commit.

[gemini-code-assist]

The default value of "0.2" for maxDiffPixelRatio is quite high for visual regression testing, as it allows up to 20% of pixels to differ before failing. For most projects, a much lower threshold (e.g., 0.01 or 0.05) is preferred to catch subtle visual regressions. If this was intended to be the color threshold (which defaults to 0.2 in Playwright), please clarify the description. Also, consider renaming the input to max-diff-pixel-ratio to match Playwright's terminology and avoid confusion with the color threshold option.

Suggested change

	| `update-snapshots` | boolean | `false` | Regenerate baseline screenshots |
	| `threshold` | string | `"0.05"` | Pixel diff tolerance (`maxDiffPixelRatio`) |

[gemini-code-assist]

According to the repository style guide, action versions should be pinned to the major version (e.g., @v0) rather than @main. This ensures stability for caller repositories.

Suggested change

	visual-regression:
	uses: apermo/reusable-workflows/.github/workflows/reusable-wp-visual-regression.yml@v0

References

1. Action versions must be pinned to major (e.g. @v4, not @v4.1.2 or a SHA). ^(link)