Preserve both Unreleased fixes and integrate the Panel geometry regression checks. Clarify the documented exception for sibling geometry inside a plain Item, addressing the non-blocking review feedback.

Validation: make check passed in the pinned CI Plasma image (881 main QtTests, 3 native-label tests, 10 native-geometry tests, 62 Python tests; no skips). Four targeted OpenGL smoke captures passed and were visually reviewed.