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/Store/StoryStore.swift

@@ -51,15 +51,14 @@ final class StoryStore: ObservableObject {
 
     // MARK: - Load
 
-    func loadStories(refresh: Bool = false) async {
-        guard !isLoading else { return }
+    @discardableResult
+    func loadStories(refresh: Bool = false) async -> Int {
+        guard !isLoading else { return 0 }
         isLoading = true
         error = nil
 
-        // Don't wipe the visible feed up-front. Only the cursor resets for a
-        // refresh; the list is replaced on success. A cancelled/failed refresh
-        // then keeps showing what we already have instead of blanking out.
         let cursor = refresh ? nil : nextCursor
+        var newCount = 0
 
         do {
             let result = try await api.fetchStories(
@@ -67,17 +66,38 @@ final class StoryStore: ObservableObject {
                 topic: selectedTopic,
                 tags: selectedTags
             )
-            stories = refresh ? result.data : stories + result.data
+            if refresh {
+                let existingIds = Set(stories.map(\.id))
+                newCount = result.data.filter { !existingIds.contains($0.id) }.count
+                stories = result.data
+            } else {
+                stories += result.data
+                newCount = result.data.count
+            }
             nextCursor = result.nextCursor
             hasMore = result.hasMore
             lastSyncedAt = Date()
+            preCacheArticlesInBackground()
         } catch let e as APIError where !e.isCancelled {
             error = e
-        } catch {
-            // Benign cancellation (network blip / superseded request): keep state.
-        }
+        } catch {}
 
         isLoading = false
+        return newCount
+    }
+
+    private func preCacheArticlesInBackground() {
+        guard let container = BackgroundRefreshManager.container else { return }
+        let snapshot = stories
+        Task.detached { await BackgroundRefreshManager.preCacheArticles(snapshot, container: container) }
+    }
+
+    /// Merge cached stories into the live feed (used when pull-refresh finds nothing new).
+    func mergeStories(_ extra: [StorySummary]) async {
+        let existing = Set(stories.map(\.id))
+        let novel = extra.filter { !existing.contains($0.id) }
+        guard !novel.isEmpty else { return }
+        stories = (stories + novel).sorted(by: StorySummary.feedOrder)
     }
 
     func loadMore() async {