In-editor revisions: Highlight changed words in the code diff

[priethor]

What

Follow up to #80314, related to #79120.

The revisions code diff now calls out individual word changes when added and removed lines are similar. When they are not, it keeps the existing full-line treatment.

Why

Line-level coloring makes small text edits hard to spot in raw markup.

How

Like the classic revisions screen, it falls back to the full-line treatment when lines are too different for a useful word diff. Because this version runs in the browser, oversized changed blocks and timed-out word comparisons use the same fallback.

Word highlights are visual only, leaving the existing row announcements unchanged. Forced-colors mode uses a strikethrough for removed words and an underline for added words.

Testing Instructions

1. Create two revisions that include a small text edit and a fully replaced line.
2. Open the revisions screen, choose Options → Code editor, and turn on Show changes.
3. Confirm the small edit is highlighted at word level and the replaced line keeps the full-line highlight.
4. Emulate forced colors and confirm word-level changes remain distinguishable.

Screenshots

Before	After

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I used Claude to help implement these changes. I guided the work, then tested and reviewed the result.

[github-actions]

Size Change: +900 B (+0.01%)

Total Size: 7.82 MB

📦 View Changed

Filename	Size	Change
build/scripts/editor/index.min.js	510 kB	+694 B (+0.14%)
build/styles/editor/style-rtl.css	31.6 kB	+58 B (+0.18%)
build/styles/editor/style-rtl.min.css	26.9 kB	+44 B (+0.16%)
build/styles/editor/style.css	31.6 kB	+60 B (+0.19%)
build/styles/editor/style.min.css	26.8 kB	+44 B (+0.16%)

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[github-actions]

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Co-authored-by: priethor <priethor@git.wordpress.org>

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