Document and verify real Clean Up execution preview for independent file/step selection

[Copilot]

The issue was that Help and tests covered only configuration-tab sample previews, not the real LTK execution preview shown by Source → Clean Up.... This PR adds deterministic SWTBot coverage and Help artifacts for independent per-step/per-file selection behavior, while keeping coordinated multi-file atomic migrations explicitly out of scope.

• Execution behavior: JFace cleanup option wiring + step semantics

  • Exposed JFACE_CLEANUP master option in the JFace cleanup tab and wired monitor/viewer-sorter/image-DPI options as slave preferences.
  • Switched JFace step descriptions from a single aggregated string to one user-facing description per enabled migration.
  • Labeled TextEditGroups with migration-specific user text (SubProgressMonitor, ViewerSorter, ImageDataProvider) instead of generic/internal identifiers.
  • Resolved step labels lazily from NLS messages to avoid enum static-init timing issues.

• Deterministic SWTBot fixture for real LTK preview

  • Extended SandboxHelpScreenshotsSWTBotTest with a real cleanup-wizard scenario that:
    • creates deterministic Java fixture sources,
    • opens the real Clean Up wizard preview page,
    • verifies one-file/multi-step preview tree labels,
    • selects different steps and asserts diff viewer content changes,
    • deselects one step and verifies only selected edits are applied,
    • runs package-level cleanup across multiple files, deselects one file, and verifies selected file changes while deselected file remains byte-identical,
    • performs undo and verifies restoration.

• Help and screenshot inventory

  • Added canonical execution-preview screenshots for:
    • single file with multiple cleanup steps,
    • selected-step diff,
    • multi-file independent selection.
  • Updated JFace usage Help to explain:
    • config-tab preview vs real execution preview,
    • independent local deselection behavior,
    • separation from coordinated multi-file atomic migrations.
  • Added a shared conceptual Help page (cleanup-preview.html) and linked it from relevant cleanup usage pages (JFace/JUnit/Int-to-Enum).
  • Updated structural Help checks to require the new screenshots and validate cross-bundle /topic/... links.

• Targeted regression coverage

  • Added JFaceCleanUpDescriptionsTest to verify:
    • one description per enabled migration,
    • master-gate behavior (JFACE_CLEANUP) controls execution requirements and exposed steps.

@Override
public String[] getStepDescriptions() {
    List<String> result = new ArrayList<>();
    if (isEnabled(JFACE_CLEANUP)) {
        computeFixSet().stream()
            .map(JfaceCleanUpFixCore::getStepDescription)
            .forEach(result::add);
    }
    return result.toArray(new String[0]);
}

• Fixes Document and verify the real Cleanup preview for local and independent changes #1452

[carstenartur]

Superseded by #1464. The consolidated PR carries the real LTK preview behavior forward and combines it with coordinated multi-file atomic preview support, product smoke coverage, Help artifacts, and release-gate verification. Closing this predecessor to avoid parallel conflicting integration branches.