Post Revisions: Migrate remaining diff imports, add CHANGELOG entries

Addresses review feedback on #77992 (findings 2, 3, 4, 10).

- `preserve-client-ids.js` and `block-compare/index.js` now import
  `diffArrays` / `diffChars` from the top-level `'diff'` package
  instead of the deep `'diff/lib/diff/<name>'` paths. v8's
  `package.json` `exports` map only wildcards `./lib/*.js` (with
  extension); the bare-folder `./lib/` mapping requires a trailing
  slash. The deep paths only resolve here because the bundler/Jest
  resolver fills in `.js` -- a future tooling change could break
  them. v8 also marks the package `sideEffects: false`, so the
  historical tree-shaking reason for the deep imports no longer
  applies. The "diff doesn't tree-shake correctly" comment in
  `block-compare/index.js` is now stale and gets removed.

- Add `### Internal` entries to `packages/editor/CHANGELOG.md` and
  `packages/block-editor/CHANGELOG.md` recording the major dependency
  bump.

Author: manzoorwanijk

packages/block-editor/CHANGELOG.md

@@ -6,6 +6,10 @@
 
 -   `BlockManager`: Add stacking context isolation to category list ([#77759](https://github.com/WordPress/gutenberg/pull/77759)).
 
+### Internal
+
+-   Updated `diff` dependency from `^4.0.2` to `^8.0.3` ([#77992](https://github.com/WordPress/gutenberg/pull/77992)).
+
 ## 15.18.0 (2026-04-29)
 
 ### Enhancements

packages/block-editor/src/components/block-compare/index.js

@@ -2,9 +2,7 @@
  * External dependencies
  */
 import clsx from 'clsx';
-// diff doesn't tree-shake correctly, so we import from the individual
-// module here, to avoid including too much of the library
-import { diffChars } from 'diff/lib/diff/character';
+import { diffChars } from 'diff';
 
 /**
  * WordPress dependencies

packages/editor/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Internal
+
+-   Updated `diff` dependency from `^4.0.2` to `^8.0.3` ([#77992](https://github.com/WordPress/gutenberg/pull/77992)).
+
 ## 14.45.0 (2026-04-29)
 
 ## 14.44.0 (2026-04-15)

packages/editor/src/components/post-revisions-preview/preserve-client-ids.js

@@ -1,7 +1,7 @@
 /**
  * External dependencies
  */
-import { diffArrays } from 'diff/lib/diff/array';
+import { diffArrays } from 'diff';
 
 /**
  * Preserves clientIds from previously rendered blocks to prevent flashing.