Fix feed jank: make seen-tracking and caching cheap

The "All" feed stuttered because every row appearing ran markSeen, which did a
disk save(), an O(n) contains over a @Query'd SeenStory array, and triggered a
full feed re-render on each insert. Now markSeen does an O(1) insert into a
non-observed reference Set, no per-row save (the container autosaves), and
SeenStory is no longer @Query'd by the view. cacheStories also does one fetch +
an in-memory index instead of a query per story.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Jarvis/Views/Home/SignalFeedView.swift

@@ -25,7 +25,6 @@ struct SignalFeedView: View {
     @Query private var cachedArticles: [CachedArticle]
     @Query private var readStories: [ReadStory]
     @Query private var savedStories: [SavedStory]
-    @Query private var seenStories: [SeenStory]
     @State private var showFeeds = false
     @State private var showSettings = false
     @State private var selectedPill: StoryPill = .all
@@ -33,6 +32,8 @@ struct SignalFeedView: View {
     /// Snapshot of already-seen story ids, captured per load so the order stays
     /// stable while scrolling (and refreshes to sink newly-seen ones).
     @State private var seenSnapshot: Set<String> = []
+    /// Reference-type set so marking a story seen on scroll doesn't re-render.
+    @State private var seenTracker = SeenTracker()
     @State private var showRead = false
 
     /// Stories that have full article content cached → eligible for the green dot.
@@ -67,13 +68,17 @@ struct SignalFeedView: View {
         filteredStories.filter { readStoryIds.contains($0.id) }
     }
 
-    private func captureSeenSnapshot() { seenSnapshot = Set(seenStories.map(\.id)) }
+    private func captureSeenSnapshot() {
+        let ids = (try? modelContext.fetch(FetchDescriptor<SeenStory>()))?.map(\.id) ?? []
+        seenSnapshot = Set(ids)
+        seenTracker.ids = seenSnapshot
+    }
 
+    /// Hot path — runs as each row scrolls in. O(1), no disk write, no re-render:
+    /// the container autosaves, and SeenStory isn't @Query'd by this view.
     private func markSeen(_ s: StorySummary) {
-        let id = s.id
-        guard !seenStories.contains(where: { $0.id == id }) else { return }
-        modelContext.insert(SeenStory(id: id))
-        try? modelContext.save()
+        guard seenTracker.ids.insert(s.id).inserted else { return }
+        modelContext.insert(SeenStory(id: s.id))
     }
 
     private func markUnread(_ s: StorySummary) {
@@ -459,24 +464,32 @@ struct SignalFeedView: View {
     // MARK: - Caching
 
     private func cacheStories(_ stories: [StorySummary]) {
+        guard !stories.isEmpty else { return }
+        // One fetch + an in-memory index, instead of a query per story.
+        let existing = (try? modelContext.fetch(FetchDescriptor<CachedStory>())) ?? []
+        var byId = Dictionary(existing.map { ($0.id, $0) }, uniquingKeysWith: { a, _ in a })
         for summary in stories {
-            let id = summary.id
-            let existing = try? modelContext.fetch(
-                FetchDescriptor<CachedStory>(predicate: #Predicate { $0.id == id })
-            )
-            if let found = existing?.first {
+            if let found = byId[summary.id] {
                 found.signalScore = summary.signalScore
                 found.sourceCount = summary.sourceCount
                 found.tags = summary.tags ?? []
                 found.updatedAt = summary.updatedAt
             } else {
-                modelContext.insert(CachedStory(from: summary))
+                let c = CachedStory(from: summary)
+                modelContext.insert(c)
+                byId[summary.id] = c
             }
         }
         try? modelContext.save()
     }
 }
 
+/// Session-seen ids in a reference type — mutating it doesn't trigger SwiftUI
+/// updates, so marking rows seen on scroll stays cheap.
+private final class SeenTracker {
+    var ids = Set<String>()
+}
+
 // Map a cached story back into a StorySummary for offline rendering.
 extension StorySummary {
     init(cached: CachedStory) {