Fix diff readability and performance for CKEditor 5

Problem/Motivation

CKEditor 5 saves content as a single, minified HTML string without line breaks (as tracked in Core issues #3351660 and #3361617). Because we cannot rely on a Core-level database formatting fix anytime soon, we must handle this formatting within the Diff module to prevent two major regressions:

• Split & Unified Fields: Because these use a line-by-line algorithm, feeding them a single line of HTML causes the engine to mark the entire field as one massive change. The diff becomes unreadable.
• Visual Inline: The engine struggles to parse and group massive, unbroken HTML strings. This causes severe performance issues on large revisions and even false positives based on some reports.

Steps to reproduce

1. Draft multiple paragraphs in a CKEditor field, preferably large samples
2. Edit the content and modify the content, again preferably with large samples
3. Compare the revisions and test the different comparison modes

Proposed resolution

Format the HTML and optimizes the engines:

• Add a formatHtmlForDiff() helper that injects a \n after closing block-level tags.
• Fix Visual Inline performance and accuracy by disabling setGroupDiffs. This forces the library to diff word-by-word instead of doing complex block grouping. Bypassing the grouping logic completely resolves the severe performance bottlenecks on large node revisions and eliminates the chunks of false highlights.
• Disable Purifier (may need further review). Disabling setPurifierEnabled helps reduce processing time in large samples. Because Drupal's Renderer pipeline already guarantees XSS safety before the Diff module sees the HTML, running HTMLPurifier again inside the diff library is redundant..

Remaining tasks

User interface changes

API changes

Data model changes