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/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