Duotone: Dedupe duotone utils into global-styles-engine

[aaronrobertshaw]

What?

The duotone helper utilities lived as two byte-for-byte copies: one in @wordpress/block-editor (components/duotone/utils.js) and a typed twin in @wordpress/global-styles-engine (utils/duotone.ts). This PR removes the block-editor copy so global-styles-engine is the single source, exposed through that package's private APIs.

Why?

global-styles-engine is meant to be the canonical home for style generation, and block-editor already depends on it and consumes its private APIs (for example getViewportBreakpoints). The duplicate copy had to be kept in sync by hand, so the two could silently drift apart. Removing it deletes that hazard with no behavior change.

How?

• Expose getDuotoneFilter, getDuotoneStylesheet, and getDuotoneUnsetStylesheet from global-styles-engine via its privateApis.
• Point block-editor at those helpers instead of the local copy, including its own getDuotoneFilter private API re-export.
• Delete the duplicated components/duotone/utils.js.

Alternative Solutions

An alternative here is that we could simply delete the unused duotone utils from the global style engine.

Notes

• No public API change; getDuotoneFilter stays available as a @wordpress/block-editor private API.
• No behavior change, no PHP changes, and no dev note needed.

Testing Instructions

1. Insert an Image block, apply a duotone filter from the toolbar, and confirm it renders in the editor and on the front end.
2. Apply both a custom duotone (two custom colors) and a preset duotone, and confirm each renders correctly.
3. Remove the duotone and confirm the filter is cleared.
4. Run npm run test:unit -- packages/block-editor/src/hooks/test/duotone.js packages/global-styles-engine/src/test/render.test.ts.

[github-actions]

Size Change: +929 B (+0.01%)

Total Size: 7.75 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-editor/index.min.js	428 kB	+25 B (+0.01%)
build/scripts/block-library/index.min.js	355 kB	+810 B (+0.23%)
build/scripts/edit-site/index.min.js	304 kB	+94 B (+0.03%)

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[github-actions]

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Co-authored-by: aaronrobertshaw <aaronrobertshaw@git.wordpress.org>
Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>

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[ramonjd]

This is testing well manually for me:

• custom duotones on blocks
• present duotones on blocks
• image global styles work as expected, overwritten by local block changes

Checked editor and frontend.