Playlist Block: Add artwork to play button

[scruffian]

What?

Adds optional playlist track artwork inside the waveform player play button.

Why?

This keeps the artwork treatment separate from the main waveform player PR so it can be reviewed independently.

How?

• Moves or creates the waveform artwork image inside the play button when an artwork URL is available.
• Adds CSS for the artwork-backed play button and contrast overlay.
• Refreshes artwork when the frontend playlist switches tracks.
• Adds unit coverage for artwork insertion, fallback creation, missing-button handling, icon contrast, and artwork removal.

Testing Instructions

• npm run test:unit packages/block-library/src/utils/test/waveform-utils.js -- --runInBand
• npm run lint:js -- packages/block-library/src/playlist/view.js packages/block-library/src/utils/waveform-utils.js packages/block-library/src/utils/test/waveform-utils.js

[github-actions]

Size Change: +1.3 kB (+0.02%)

Total Size: 7.68 MB

📦 View Changed

Filename	Size	Change
build/modules/block-library/playlist/view.min.js	17.2 kB	+177 B (+1.04%)
build/scripts/block-library/index.min.js	334 kB	+154 B (+0.05%)
build/styles/block-library/playlist/style-rtl.css	2.45 kB	+133 B (+5.74%)	🔍
build/styles/block-library/playlist/style-rtl.min.css	2.33 kB	+121 B (+5.47%)	🔍
build/styles/block-library/playlist/style.css	2.46 kB	+138 B (+5.95%)	🔍
build/styles/block-library/playlist/style.min.css	2.34 kB	+121 B (+5.46%)	🔍
build/styles/block-library/style-rtl.css	22.5 kB	+120 B (+0.54%)
build/styles/block-library/style-rtl.min.css	19 kB	+108 B (+0.57%)
build/styles/block-library/style.css	22.6 kB	+121 B (+0.54%)
build/styles/block-library/style.min.css	18.9 kB	+106 B (+0.56%)

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[github-actions]

Flaky tests detected in b917354.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/28859233741
📝 Reported issues:

• [Flaky Test] can use appender in site editor sidebar list view #79632 in /test/e2e/specs/site-editor/navigation-sidebar-list-view.spec.js
• [Flaky Test] three users concurrently edit a large post with diverse blocks (RTC WebSocket) #79926 in /test/e2e/specs/editor/collaboration/collaboration-stress.spec.ts

[scruffian]

Closing in favour of #79938