fix: stale-news control — repair clientRank's age source, add isFossil backstop

clientRank already existed specifically to stop stale high-scorers from
leading the feed, but computed age from updatedAt — the same timestamp
that keeps resetting whenever the backend attaches any article to a
cluster (BACKLOG #15). A fossil's age always read as ~0, silently
defeating the whole mechanism. Fixed to use firstSeenAt.

clientRank's decay is soft (max 20% penalty) and isn't always enough
against a high signalScore alone, so added StorySummary.isFossil (>5
days old by firstSeenAt) as a hard backstop: mainStories and
makeDigest now sink fossils below current stories, so one can never
take the hero/lead slot regardless of score. Still shown in the feed,
just not in the most prominent position.

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

Jarvis/Views/Home/SignalFeedView.swift

@@ -51,12 +51,16 @@ struct SignalFeedView: View {
         return matched
     }
 
-    /// The main feed: unread stories, with already-seen ones sunk below fresh ones.
+    /// The main feed: unread stories, with already-seen ones sunk below fresh
+    /// ones, and fossils (old by firstSeenAt, see StorySummary.isFossil) sunk
+    /// below everything — a high signalScore alone can't make old news lead.
     private var mainStories: [StorySummary] {
         let notRead = filteredStories.filter { !readStoryIds.contains($0.id) }
-        let fresh = notRead.filter { !seenSnapshot.contains($0.id) }
-        let seen = notRead.filter { seenSnapshot.contains($0.id) }
-        return fresh + seen
+        let current = notRead.filter { !$0.isFossil }
+        let fossils = notRead.filter { $0.isFossil }
+        let fresh = current.filter { !seenSnapshot.contains($0.id) }
+        let seen = current.filter { seenSnapshot.contains($0.id) }
+        return fresh + seen + fossils
     }
 
     /// Read stories, tucked into the collapsible "Read" shelf.
@@ -72,8 +76,13 @@ struct SignalFeedView: View {
     /// per-section targeted fetches held in store.sectionSupplement.
     private func makeDigest(sections: [NewsSection]) -> (top: [StorySummary], sections: [(NewsSection, [StorySummary])]) {
         let unread = filteredStories.filter { !readStoryIds.contains($0.id) }
-        let top = Array(unread.prefix(5))
-        var pool = Array(unread.dropFirst(5))
+        // Fossils (old by firstSeenAt) sink below current stories so the lead
+        // slot is never a high-scoring but stale story — see StorySummary.isFossil.
+        let current = unread.filter { !$0.isFossil }
+        let fossils = unread.filter { $0.isFossil }
+        let reordered = current + fossils
+        let top = Array(reordered.prefix(5))
+        var pool = Array(reordered.dropFirst(5))
         var usedIds = Set(top.map(\.id))
         var out: [(NewsSection, [StorySummary])] = []
         for section in sections {

Jarvis/Views/Shared/Theme.swift

@@ -269,14 +269,32 @@ enum AppearanceMode: String, CaseIterable {
 //
 // Effect: a score-80 story from 1 hour ago (rank ≈ 79.8) beats a score-82 story
 // from 48 hours ago (rank ≈ 71.4). Pure score still dominates for same-age stories.
+//
+// Age is measured from `firstSeenAt`, not `updatedAt` — the backend bumps
+// `updatedAt` every time it attaches another article to a cluster, however
+// loosely related (see BACKLOG #15), so a story can read as "just now" while
+// being weeks old by `firstSeenAt`. Using `updatedAt` here previously
+// neutralized the whole point of this decay: a fossil story's age always
+// computed near zero.
 
 extension StorySummary {
     var clientRank: Double {
-        let ageHours = max(0.0, -updatedAt.timeIntervalSinceNow) / 3600.0
+        let ageHours = max(0.0, -(firstSeenAt ?? updatedAt).timeIntervalSinceNow) / 3600.0
         let decay = 1.0 / (1.0 + ageHours / 24.0)
         return Double(signalScore) * (0.80 + 0.20 * decay)
     }
 
+    /// Even with the fix above, this decay is soft (max 20% penalty) — a
+    /// high enough signalScore can still outrank fresher, lower-scored
+    /// stories. `isFossil` is a hard backstop views use to keep genuinely
+    /// old stories out of the lead/hero slot regardless of score.
+    static let fossilThresholdDays: Double = 5
+
+    var isFossil: Bool {
+        let ageSeconds = -(firstSeenAt ?? updatedAt).timeIntervalSinceNow
+        return ageSeconds > StorySummary.fossilThresholdDays * 86400
+    }
+
     static func feedOrder(_ a: StorySummary, _ b: StorySummary) -> Bool {
         let diff = a.clientRank - b.clientRank
         // Stable tiebreak for nearly-equal ranks: newer ID wins (IDs are