Bound market memory and let large areas page instead of loading everything:
- DiscoveryQuery gains an until cursor; OfferTransport gains discoverPage()
(one-shot, EOSE-bounded, newest-first) alongside the existing live discover()
stream, which now accepts since so it only carries NEW offers going forward.
- NostrOfferTransport.discoverPage sorts by created_at desc and derives the
next cursor from the oldest event seen (not the oldest type-matched one, so
filtering never skips a page).
- OffersCubit: discover() fetches the first page + opens a since=now live sub;
loadNextPage() pages further back; the in-memory list is capped at 400
offers (each can carry a ~40KB inline photo thumbnail, so unbounded growth in
a busy area was a real OOM risk).
- market_screen: infinite scroll via a scroll-position trigger + footer
spinner, instead of a single unbounded ListView.
- Added discoverPage unit tests (commons_core) and cubit pagination tests
(first page/cursor, accumulation, cap, cross-page dedup).