Aliases list a copy's picture under the copy's own id for builds from
before pictures had their own id. Every reader of page:getFrame and
page:getElementSlice keeps picture ids (and looks pictures up by them), so
the read in parts now always sends them and sends no aliases. Rebuilding
aliases across slices would also have had to follow chains of them in page
order, which independent slices can't.

getSnapshot reads its assets before sorting elements and lineups again, as
on main, so its answer keeps its order for every input.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>