Regression coverage for the safety-sensitive part of keyboard copy/paste:
nothing previously tested that Ctrl/Cmd+V regenerates node ids via
regenerateTreeIds before actions.addNodeTree -- the exact logic whose
absence caused a duplicate-id corruption bug. Also covers sibling-parent
targeting (with ROOT fallback), the empty-clipboard no-op, and the
existing input/contentEditable focus guard.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>