feat(next): export Versions diff surface as @payloadcms/next/views/diff

[tsemachh]

Closes #16496

Summary

Adds a public, additive subpath export @payloadcms/next/views/diff that re-exports the field-by-field diff rendering surface today living privately under packages/next/src/views/Version/RenderFieldsToDiff/.

This lets plugin and integrator code build "review changes" / custom diff UIs (drawers, dashboards, notifications, PR-bot integrations, etc.) without forking or vendoring Payload's diff components or deep-importing from @payloadcms/next/dist/....

Public surface

import {
  RenderDiff,
  buildVersionFields,
  RenderVersionFieldsToDiff,
  DiffCollapser,
  defaultDiffComponents, // alias of diffComponents
  diffComponents,
  countChangedFields,
  countChangedFieldsInRows,
  fieldHasChanges,
  getFieldsForRowComparison,
  type BuildVersionFieldsArgs,
} from '@payloadcms/next/views/diff'

Changes

• New packages/next/src/exports/views/diff.ts — pure re-export entry, no logic.
• packages/next/package.json — adds "./views/diff" entry to both exports and publishConfig.exports, mirroring the shape of the existing "./views" entry.

Why not also point Version/index.tsx at the new public path?

The original plan considered making the internal Versions view import from @payloadcms/next/views/diff (or the relative exports/views/diff.js) so external and internal usage shared one source of truth. The repository's payload/no-imports-from-exports-dir lint rule explicitly forbids that pattern. The single-source-of-truth guarantee still holds because the new entry file is a pure re-export of the same internal modules Version/index.tsx consumes — there is no second copy of any symbol.

Backward compatibility

• 100% additive: no existing import path changes, no symbol is renamed or removed.
• Version/index.tsx is not modified — it continues to import from ./RenderFieldsToDiff/index.js as before.
• No SCSS changes; existing per-field SCSS already ships in dist/.
• No changes to payload, @payloadcms/translations, or any other package.

Tests

This PR is a pure re-export with no new behavior. The existing test/versions/ suite already exercises every symbol exposed here through the built-in Versions view. No new test was added.

If maintainers prefer a smoke test asserting the subpath resolves and exposes the listed names, happy to add one in test/_community/exports.test.ts (or a similar location) — let me know.

Concrete consumer

A planned @shefing/review-button plugin will use this surface to render a per-document diff in a side drawer. Without this PR, the plugin would have to vendor RenderFieldsToDiff/ and re-vendor on every Payload release.

Companion proposal

A separate issue (#16497) and forthcoming PR add a plugin-friendly config.admin.serverFunctions registry so plugins built on top of this export can wire themselves in without forcing integrators to edit (payload)/layout.tsx. This PR stands alone and does not depend on it.