tw ← Tiptap WYSIWYG: comparison/diff mode (CMS-2649)

[alvarosabu]

Scope

Adds comparison/diff mode (content versioning and revision diffs) to the new Tiptap WYSIWYG editor. The comparison view already special-cases input-rich-text-html fields (use-comparison.ts computes the diff and feeds pre-marked HTML as the field value); this PR adds the interface side that renders it.

What's changed

• New ComparisonDiff mark (extensions/comparison-diff.ts): lets span.comparison-diff--added / --removed spans (produced by use-comparison-diff.ts) survive schema parsing. Registered only in comparison mode — the normal editing schema still strips diff spans, so they can never leak into saved content.
• New comparisonMode / comparisonSide props on the interface (passed generically by form-field-interface.vue). comparisonSide is API surface only — side switches flow in as value changes.
• Comparison mode renders read-only with the normal (non-dimmed) look: no toolbar, no table bubble menu, no soft-length counter (the diff-marked value inflates the count).
• Added/removed highlight styles on the theme success/danger tokens.
• Unit tests: mark round-trip for added/removed spans across block contexts, a guard that the normal schema strips them, and component-level wiring tests (read-only, chrome hidden, counter hidden, spans intact).

Potential Risks / Drawbacks

• ComparisonDiff joins the schema at editor construction based on comparisonMode. The comparison view mounts the interface fresh per comparison, so deciding at construction is safe; a hypothetical runtime toggle of the prop would not re-register the extension.
• The diff runs on stored HTML upstream, before the interface decodes page-break markers — content containing page breaks may render the marker oddly inside the comparison view (upstream reconstructComparisonHtml concern, not introduced here).

Tested Scenarios

• Diff spans survive round-trip in paragraphs, headings, list items, nested bold, and links — with the mark registered.
• Without the mark (normal editing), diff spans are stripped.
• Comparison mode: editor read-only, toolbar/bubble-menu/soft-length counter absent, non-editable class applied, diff-marked value rendered intact.
• Normal mode unaffected: editable, toolbar present.
• Verified visually in the studio against a versioned item (base side shows removed runs in red, incoming in green; unchanged blocks unmarked).
• Full input-rich-text-html suite (256 tests) passes.

Review Notes / Questions

• Targets the feature branch feat/wysiwyg-tiptap, not main.
• The base/incoming diff attribution itself (what gets marked) is upstream comparison-view logic shared with the legacy editor; this PR only renders what it is given.

Checklist

• Added or updated tests
• Documentation PR created here or not required — not required
• OpenAPI package PR created here or not required — not required

---

Closes CMS-2649

[linear-code]

CMS-2649

[AlexGaillard]

🆒🫘