add prev/next arrow buttons (only in gallery mode with >1 image) and
ArrowLeft/ArrowRight keyboard handling. switching uses a 150ms
fade-out -> swap src -> fade-in (no fly, matching Medium). originNode
is updated to the new image so close/scroll-close flies back to the
right place; openScrollY is reset to keep scroll-close baseline valid.
bindInteractions() was only called from start(), which waits for the
image load event. during the load window Esc/scroll/backdrop-click did
nothing. bind right after appending the overlay so the lightbox is
closable throughout the load.
click a .blur-img to open: record its viewport rect, the lightbox img
starts at that position/size and transforms to centered (Medium-style
origin-aware zoom). close flies back to the live originNode rect.
scroll-to-close: while open, any window scroll drives the image back
to its (moving) position via per-frame rect tracking + linear
interpolation; the article keeps its scroll position on close.
Esc + click-on-backdrop close; click-on-image does not close.
prefers-reduced-motion falls back to fade-only / immediate close.
