Re-deliver live-run frames across a WatchConnectivity drop

Stage the latest live-run frame (and the terminal .ended marker) in a
depth-1 latest-wins slot on both bridges, and flush it on reachability/
activation restore. A brief connectivity drop no longer desyncs the
iPhone/Watch run mirror; frames carry a wall-clock anchor so a late
re-send self-corrects rather than reading as stale.

Claude-Session: https://claude.ai/code/session_01A9CfUa4E9Zd5swfoNsYPs7

Workouts Watch App/Connectivity/WatchConnectivityBridge.swift

@@ -28,6 +28,13 @@ final class WatchConnectivityBridge: NSObject {
     /// side can drop a stale / out-of-order delivery (see `LiveProgress.version`).
     private var liveVersion = 0
 
+    /// The latest live-run message we haven't confirmed reached the phone (depth 1, latest-wins).
+    /// Staged regardless of reachability and re-sent by `flushLive()` when reachability/activation
+    /// returns, so a brief WatchConnectivity drop doesn't desync the mirror. A newer frame — or the
+    /// terminal `.ended` — replaces it, so we never deliver stale state. Frames carry an absolute
+    /// wall-clock anchor, so a late re-send self-corrects on arrival rather than reading as stale.
+    private var pendingLive: PendingLive?
+
     /// The latest live-run frame the *phone* sent, for the run we currently have open to apply
     /// (ephemeral; nil when the phone isn't driving). The watch's `ExerciseProgressView` reads
     /// this to follow a phone-driven transition; it's never persisted.
@@ -86,24 +93,47 @@ final class WatchConnectivityBridge: NSObject {
         sendToPhone(doc)
     }
 
-    // MARK: - Live run mirror (ephemeral; reachable-only)
+    // MARK: - Live run mirror (ephemeral; coalesced redelivery)
 
-    /// Broadcast where the run flow currently is, so a propped-up iPhone can mirror it. Sent
-    /// over `sendMessage` only when the phone is reachable — this is throwaway presence, so
-    /// there's no guaranteed-delivery fallback (a queued frame would be stale on arrival).
+    /// Broadcast where the run flow currently is, so a propped-up iPhone can mirror it. Staged as
+    /// the latest pending frame and sent when the phone is reachable; if it's unreachable the frame
+    /// is held and re-sent on reconnect (`flushLive`). Because frames are full state snapshots with
+    /// a wall-clock anchor, holding only the newest one (depth 1) and self-correcting its timers on
+    /// arrival means a re-send is never stale.
     func sendLiveProgress(_ frame: LiveProgress) {
-        guard let session, session.activationState == .activated, session.isReachable else { return }
+        guard let session, session.activationState == .activated else { return }
         liveVersion += 1
         var stamped = frame
         stamped.version = liveVersion
-        session.sendMessage(WCPayload.encodeLiveProgress(stamped), replyHandler: nil, errorHandler: { _ in })
+        pendingLive = .progress(stamped)
+        flushLive()
     }
 
-    /// Tell the phone to stop mirroring this run (the user left the progress flow).
+    /// Tell the phone to stop mirroring this run (the user left the progress flow). Staged like a
+    /// frame so a drop at the moment the run ends doesn't strand the phone's follower cover — the
+    /// terminal marker supersedes any pending progress and is re-sent on reconnect.
     func sendLiveEnded(workoutID: String, logID: String) {
-        guard let session, session.activationState == .activated, session.isReachable else { return }
-        session.sendMessage(WCPayload.encodeLiveEnded(workoutID: workoutID, logID: logID),
-                            replyHandler: nil, errorHandler: { _ in })
+        guard let session, session.activationState == .activated else { return }
+        pendingLive = .ended(workoutID: workoutID, logID: logID)
+        flushLive()
+    }
+
+    /// (Re)send the staged live-run message if the phone is reachable. Called on each new frame and
+    /// whenever reachability/activation is restored. Leaves it staged on failure; a newer frame or
+    /// `.ended` supersedes it, so we never deliver stale state. The error handler runs on
+    /// WatchConnectivity's background queue, so it must be nonisolated (@Sendable) — an
+    /// inherited-@MainActor closure would trap (swift_task_checkIsolated) there.
+    private func flushLive() {
+        guard let session, let pending = pendingLive,
+              session.activationState == .activated, session.isReachable else { return }
+        let payload: [String: Any]
+        switch pending {
+        case .progress(let frame):
+            payload = WCPayload.encodeLiveProgress(frame)
+        case .ended(let workoutID, let logID):
+            payload = WCPayload.encodeLiveEnded(workoutID: workoutID, logID: logID)
+        }
+        session.sendMessage(payload, replyHandler: nil, errorHandler: { @Sendable _ in })
     }
 
     /// Apply a live-run frame the phone sent. Drops a stale one for the same run, and catches
@@ -126,8 +156,13 @@ final class WatchConnectivityBridge: NSObject {
         guard let session, session.activationState == .activated else { return }
         let payload = WCPayload.encodeWorkoutUpdate(doc)
         if session.isReachable {
-            session.sendMessage(payload, replyHandler: nil, errorHandler: { _ in
-                session.transferUserInfo(payload)   // fall back to guaranteed delivery
+            // The error handler runs on WatchConnectivity's background queue, so it must be
+            // nonisolated (@Sendable) or it would trap (swift_task_checkIsolated). Hop back to the
+            // MainActor to fall back to guaranteed delivery; `doc` is Sendable, the payload isn't.
+            session.sendMessage(payload, replyHandler: nil, errorHandler: { @Sendable _ in
+                Task { @MainActor in
+                    _ = self.session?.transferUserInfo(WCPayload.encodeWorkoutUpdate(doc))
+                }
             })
         } else {
             session.transferUserInfo(payload)
@@ -168,7 +203,16 @@ final class WatchConnectivityBridge: NSObject {
 
 extension WatchConnectivityBridge: WCSessionDelegate {
     nonisolated func session(_ session: WCSession, activationDidCompleteWith activationState: WCSessionActivationState, error: Error?) {
-        Task { @MainActor in self.requestSync() }
+        Task { @MainActor in
+            self.requestSync()
+            self.flushLive()   // deliver any frame staged before the session was ready
+        }
+    }
+
+    nonisolated func sessionReachabilityDidChange(_ session: WCSession) {
+        if session.isReachable {
+            Task { @MainActor in self.flushLive() }   // catch the phone up on the latest run state after a reconnect
+        }
     }
 
     /// Live-run frames arrive as messages (reachable-only), distinct from the latest-wins
@@ -205,3 +249,10 @@ extension WatchConnectivityBridge: WCSessionDelegate {
         }
     }
 }
+
+/// The single staged live-run message awaiting (re)delivery to the phone — see `pendingLive`.
+/// One slot, latest-wins: a newer progress frame or the terminal `.ended` replaces whatever's held.
+private enum PendingLive {
+    case progress(LiveProgress)
+    case ended(workoutID: String, logID: String)
+}