A decoded 64 MP original (Sony A7R VI: 9984×6656 linear float, 797 MB) is bigger than the
default cache share (half of 1.5 GiB). MediaCache::enforce_budget evicted it as soon as the
render handed it over, so every pan of a 1:1 view and every slider move while zoomed decoded
the raw again (and lost the shared GPU copy, keyed by the CPU Arc).
The frontend now tells the cache which photo Detail shows (MediaCache::set_on_screen, set per
frame in panels::compare::prepare); that photo's original is not evicted for the budget and is
let go as soon as another photo or view is shown. Keying it on "no newer preview of another
photo" instead does not work: the neighbour prefetch stores previews after the original.