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 {