Stabilize feed order with a deterministic signal-score tiebreaker

Equal-score stories reshuffled on every pull-to-refresh because the sort
was unstable. Add StorySummary.feedOrder (signalScore desc, then id desc,
mirroring the backend's signal_score DESC, id DESC) and use it for the feed,
the WebSocket re-sort, and the Saved/Search tabs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Jarvis/Views/SecondaryTabs.swift

@@ -88,7 +88,7 @@ struct SavedView: View {
     @Query private var readStories: [ReadStory]
 
     private var stories: [StorySummary] {
-        cachedStories.map(StorySummary.init(cached:)).sorted { $0.signalScore > $1.signalScore }
+        cachedStories.map(StorySummary.init(cached:)).sorted(by: StorySummary.feedOrder)
     }
 
     var body: some View {
@@ -133,7 +133,7 @@ struct SearchView: View {
         return store.stories
             .filter { $0.headline.localizedCaseInsensitiveContains(query)
                    || $0.summary.localizedCaseInsensitiveContains(query) }
-            .sorted { $0.signalScore > $1.signalScore }
+            .sorted(by: StorySummary.feedOrder)
     }
 
     var body: some View {