feat: offline article caching + smart pull-to-refresh reshuffle

Offline caching:
- BackgroundRefreshManager.preCacheArticles() fetches top article per story
  (score ≥60, up to 20 stories) and stores as CachedArticle in SwiftData
- Runs after every BG fetch AND after every foreground sync via StoryStore
- Articles are available in ArticleReaderView offline without ever opening them

Pull-to-refresh reshuffle when nothing new:
- loadStories(refresh:) now returns newCount (genuinely new story IDs)
- If server returns 0 new stories, feed merges in any BG-cached stories not
  already in the live feed, then re-sorts by signal score (reshuffle)
- seenSnapshot is always cleared so everything re-ranks fresh

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Jarvis/Views/Home/SignalFeedView.swift

@@ -251,11 +251,20 @@ struct SignalFeedView: View {
         .animation(.snappy, value: readStories.count)
         .onAppear { captureSeenSnapshot() }
         .refreshable {
-            // Reshuffle: clear the seen snapshot so all stories re-rank by signal
-            // score, and any stories cached by background refresh float to the top.
             seenSnapshot = []
             seenTracker.ids = []
-            await store.loadStories(refresh: true)
+            let new = await store.loadStories(refresh: true)
+            // Nothing new from server — pull in any stories cached by background
+            // refresh that aren't already in the live feed, then reshuffle.
+            if new == 0 {
+                let liveIds = Set(store.stories.map(\.id))
+                let extra = cachedStories
+                    .filter { !liveIds.contains($0.id) }
+                    .map(StorySummary.init(cached:))
+                if !extra.isEmpty {
+                    await store.mergeStories(extra)
+                }
+            }
         }
     }