Transcript Incremental Projection

Nucleic-Session: 0FFD007B-0696-4517-9429-129C7B0FD5AC
Co-authored-by: Nucleic <[email protected]>

NucleicRemote/NucleicRemote/Models/RemoteStore.swift

@@ -812,6 +812,69 @@ final class RemoteStore: ObservableObject {
         }
     }
 
+    // MARK: - Background transcript prefetch
+
+    /// Warm the offline transcript cache for recent sessions *before* they're ever opened. A
+    /// fresh open otherwise starts from an empty transcript and waits on the host's snapshot, so
+    /// the view renders at the top and visibly drops to the end as history lands; a session
+    /// opened before seeds instantly from `SessionCache` and opens at its end. Prefetching runs
+    /// the same `fetchTranscript` flow the open path uses — one session at a time, newest first,
+    /// pulling only what the cache is missing — so every recent session opens like a warm one.
+    private var prefetchQueue: [SessionID] = []
+    private var prefetchInFlight: SessionID?
+    /// The summary `lastSeq` each session was last prefetched (or attempted) at, so a session is
+    /// re-queued only after it has moved on — not on every aggregate rebuild.
+    private var prefetchedSeq: [SessionID: UInt64] = [:]
+
+    /// Rebuild the prefetch queue from the freshest summaries. Called when the session list
+    /// actually changes (connect, session churn); cheap when nothing needs pulling.
+    private func scheduleTranscriptPrefetch() {
+        guard !demoMode else { return }
+        prefetchQueue = sessions
+            .filter { !$0.archived && $0.sessionID != openSessionID && $0.sessionID != prefetchInFlight }
+            .sorted { $0.updatedAt > $1.updatedAt }
+            .prefix(8)
+            .filter { $0.lastSeq > (prefetchedSeq[$0.sessionID] ?? 0) }
+            .map(\.sessionID)
+        pumpTranscriptPrefetch()
+    }
+
+    /// Start the next queued prefetch if none is in flight. Serial on purpose — background work
+    /// must trickle behind the live stream, not contend with it.
+    private func pumpTranscriptPrefetch() {
+        guard prefetchInFlight == nil, !prefetchQueue.isEmpty else { return }
+        let id = prefetchQueue.removeFirst()
+        guard let summary = sessions.first(where: { $0.sessionID == id }),
+              let conn = connection(owningSession: id), conn.canPrefetchTranscript
+        else {
+            // Gone / connection busy or incapable — skip; a later list update re-queues it.
+            pumpTranscriptPrefetch()
+            return
+        }
+        prefetchInFlight = id
+        // Mark the attempt at this watermark now, so an empty/unavailable result doesn't
+        // re-queue in a loop; the session re-qualifies once its lastSeq advances.
+        prefetchedSeq[id] = summary.lastSeq
+        Task { [weak self] in
+            // Pull only the gap beyond what's cached; a cold session is bounded to the same
+            // tail window the cache would keep anyway.
+            let cachedLast = await SessionCache.loadEvents(id).last?.seq ?? 0
+            guard let self else { return }
+            if cachedLast >= summary.lastSeq {
+                self.prefetchInFlight = nil  // cache already current
+                self.pumpTranscriptPrefetch()
+                return
+            }
+            let coldFloor = summary.lastSeq > UInt64(SessionCache.eventLimit)
+                ? summary.lastSeq - UInt64(SessionCache.eventLimit) : 0
+            let afterSeq = cachedLast > 0 ? cachedLast : coldFloor
+            if !conn.prefetchTranscript(id, afterSeq: afterSeq) {
+                self.prefetchInFlight = nil  // couldn't start (connection changed) — move on
+                self.pumpTranscriptPrefetch()
+            }
+        }
+    }
+
     // MARK: - Streaming delta coalescing
 
     /// Streaming transcript deltas arrive one wire frame at a time — often dozens per second