Add atomic coordinated Cleanup preview changes

[carstenartur]

Summary

Adds an optional, dependency-free contract for presenting a coordinated multi-file Cleanup migration as one atomic LTK preview entry.

• cleanups may expose getCoordinatedCleanUpPreview(IJavaProject) and return stable candidate metadata, affected compilation units, a user-facing name/description, and safety details;
• CleanUpRefactoring groups the ordinary per-file changes for those units into a unique non-text CoordinatedCleanUpChange;
• the wrapper is rendered as one top-level checkbox instead of exposing unsafe file/edit-group checkboxes;
• ordinary local cleanup changes in unrelated files retain the standard fine-grained LTK preview;
• overlapping coordinated candidates are merged into one safe atomic component;
• validation and direct perform(...) both fail closed if internal changes are partially disabled programmatically;
• apply and undo still delegate to the standard LTK CompositeChange machinery;
• original GroupCategorySet metadata is preserved while cleanup changes are merged.

Tests

CoordinatedCleanUpPreviewTest covers:

• one coordinated two-file candidate plus an independently selectable local file change;
• atomic enable/disable propagation;
• fatal validation and direct-perform rejection for a partial nested selection before resources are modified;
• atomic apply and byte-exact undo across both files;
• safe merging of overlapping coordinated candidate IDs.

The productive source and test are listed in .github/fork-specific-files.txt, so the fork synchronization workflow cannot silently remove the contract.

Refs #96
Refs carstenartur/sandbox#1451