{# THE STAGE MACHINERY (r2c, shared by R3). One copy, behind a stated interface, used by the review (one stage) and compare (two). It DEFINES two things and binds nothing by itself: BoothMode.bind({toggle, fit, one, onChange}) page level, once BoothStage.attach(stageEl, {img, onSettle}) once per stage A page binds BoothMode ONLY when at least one of its stages is an image (the review's rule): two videos get no toggle. No key is bound here — the review gains none, and compare's `Z` is compare's own (r3 C4, INV-6). No ResizeObserver here either: each page owns its own, so the review's stays in view.html with its mutation row. #}