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/Shared/Theme.swift

@@ -160,3 +160,14 @@ struct ArticleRoute: Hashable {
     let storyId: String
     let parentHeadline: String
 }
+
+// MARK: - Stable feed ordering
+
+extension StorySummary {
+    /// Deterministic ranking: signal score descending, then id descending — the
+    /// same order the backend uses (`signal_score DESC, id DESC`). Without the
+    /// id tiebreaker, equal-score stories reshuffle on every (unstable) re-sort.
+    static func feedOrder(_ a: StorySummary, _ b: StorySummary) -> Bool {
+        a.signalScore != b.signalScore ? a.signalScore > b.signalScore : a.id > b.id
+    }
+}