perf(market): paginate Nostr offer discovery, infinite scroll, memory cap

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).

apps/app_seeds/lib/ui/market_screen.dart

@@ -414,23 +414,44 @@ class MarketBody extends StatelessWidget {
                           ),
                         ],
                       )
-                    : ListView.separated(
-                        physics: const AlwaysScrollableScrollPhysics(),
-                        padding: const EdgeInsets.all(16),
-                        itemCount: visible.length,
-                        separatorBuilder: (_, _) => const SizedBox(height: 12),
-                        itemBuilder: (context, i) {
-                          final o = visible[i];
-                          final mine = o.authorPubkeyHex == selfPubkey;
-                          return _OfferCard(
-                            offer: o,
-                            mine: mine,
-                            onTap: () async {
-                              await context.push('/market/offer', extra: o);
-                              await onOfferClosed?.call();
-                            },
-                          );
+                    : NotificationListener<ScrollNotification>(
+                        // Page in older offers as the list nears its end, so a
+                        // large area streams in on demand rather than all at
+                        // once. The cubit guards against overlapping loads.
+                        onNotification: (n) {
+                          if (state.canLoadMore &&
+                              !state.loadingMore &&
+                              n.metrics.axis == Axis.vertical &&
+                              n.metrics.extentAfter < 500) {
+                            cubit.loadNextPage();
+                          }
+                          return false;
                         },
+                        child: ListView.separated(
+                          physics: const AlwaysScrollableScrollPhysics(),
+                          padding: const EdgeInsets.all(16),
+                          // A trailing spinner row while the next page loads.
+                          itemCount: visible.length + (state.loadingMore ? 1 : 0),
+                          separatorBuilder: (_, _) => const SizedBox(height: 12),
+                          itemBuilder: (context, i) {
+                            if (i >= visible.length) {
+                              return const Padding(
+                                padding: EdgeInsets.symmetric(vertical: 16),
+                                child: Center(child: CircularProgressIndicator()),
+                              );
+                            }
+                            final o = visible[i];
+                            final mine = o.authorPubkeyHex == selfPubkey;
+                            return _OfferCard(
+                              offer: o,
+                              mine: mine,
+                              onTap: () async {
+                                await context.push('/market/offer', extra: o);
+                                await onOfferClosed?.call();
+                              },
+                            );
+                          },
+                        ),
                       ),
               ),
             ),