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/Store/StoryStore.swift

@@ -139,7 +139,7 @@ final class StoryStore: ObservableObject {
             createdAt: old.createdAt
         )
         stories[idx] = updated
-        stories.sort { $0.signalScore > $1.signalScore }
+        stories.sort(by: StorySummary.feedOrder)
     }
 
     private func removeStory(id: String) {