Transcript Incremental Projection

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

NucleicRemote/NucleicRemote/Models/HostConnection.swift

@@ -692,6 +692,39 @@ final class HostConnection {
         send(.fetchTranscript(TranscriptFetch(requestID: requestID, sessionID: sessionID, afterSeq: 0)))
     }
 
+    /// Whether a background transcript prefetch can start now: live, the host can serve
+    /// transcript fetches, and no prefetch is already in flight (they run one at a time).
+    var canPrefetchTranscript: Bool {
+        connectivity.isLive && capabilities.canSyncTranscripts && client != nil && prefetchRequestID == nil
+    }
+
+    /// Fetch a session's transcript in the background — no subscribe, no open — so the offline
+    /// cache is warm before the user ever opens the session. `afterSeq` bounds the pull to what
+    /// the cache is missing. The result (all chunks, merged) arrives via
+    /// `callbacks.transcriptPrefetched`; returns false when it can't start (caller retries on a
+    /// later session-list update).
+    @discardableResult
+    func prefetchTranscript(_ sessionID: SessionID, afterSeq: UInt64) -> Bool {
+        guard canPrefetchTranscript else { return false }
+        let requestID = UUID().uuidString
+        prefetchRequestID = requestID
+        prefetchSessionID = sessionID
+        prefetchEvents = []
+        send(.fetchTranscript(TranscriptFetch(requestID: requestID, sessionID: sessionID, afterSeq: afterSeq)))
+        return true
+    }
+
+    /// Settle the in-flight prefetch: hand what arrived to RemoteStore (nothing on
+    /// unavailable/disconnect) and clear the slot so the next queued prefetch can start.
+    private func finishPrefetch(delivering: Bool) {
+        guard let sessionID = prefetchSessionID else { return }
+        let events = delivering ? prefetchEvents : []
+        prefetchRequestID = nil
+        prefetchSessionID = nil
+        prefetchEvents = []
+        callbacks.transcriptPrefetched(sessionID, events)
+    }
+
     /// The device's network path changed (Wi-Fi ⇄ cellular, joined/left a network). React now rather
     /// than waiting for a zombie LAN socket to time out or the reconnect backoff to elapse — this is
     /// what makes the transport switch feel immediate. Only reconnect-managed hosts (a pinned host)