Anchor auto-advanced phases at the boundary, not at tick time

The rest/timed-work countdown deadline is shared by both devices, but the
page flip crossing it is a local ticker event — and stamping the *next*
phase's anchor at Date() when that event finally ran baked a sleeping
watch's lateness (throttled wrist-down ticker) into its next count-up,
leaving the two devices permanently offset with nothing on the wire to
correct.

Auto-advances now chain the anchor instead: the finished phase's computed
end (passed out of CountdownPhaseView) becomes the next page's PageAnchor,
with the next window derived from it via liveSnapshot(for:at:). A device
arbitrarily late to a boundary shows exactly what the on-time device shows,
and a stack of missed boundaries fast-forwards itself — each chained page
lands already-elapsed and advances on its own next tick, skipping the
start/stop haptics for boundaries that passed while asleep (only a
just-crossed boundary buzzes).

The remoteAnchor* fields are generalized into one PageAnchor (remote frames
and chained auto-advances are the same concept: a page whose timer counts
from a known instant); the phone's Live Activity emit honors it unchanged.

Workouts Watch App/Views/ExerciseProgressView.swift

@@ -71,13 +71,19 @@ struct ExerciseProgressView: View {
     /// durable echo of our own progress (or of a frame we followed) never triggers a jump.
     @State private var knownStateIndex: Int
 
-    /// When a remote frame drove us to a page, that page's timer anchors to the *sender's*
-    /// wall clock (`remoteAnchorStart`/`End`) instead of local `now` — so delivery latency
-    /// can't desync the mirror, the way the old read-only mirror counted off the frame's
-    /// anchors. Scoped to `remoteAnchorPage`; any local transition to another page self-anchors.
-    @State private var remoteAnchorPage: Int?
-    @State private var remoteAnchorStart: Date?
-    @State private var remoteAnchorEnd: Date?
+    /// A page reached with a *known* wall-clock anchor counts its timer from that anchor
+    /// instead of local `now`. Two sources: a remote frame (the sender's clock, so delivery
+    /// latency can't desync the mirror) and a chained auto-advance (the finished phase's
+    /// computed end, so a ticker that slept through the boundary — wrist-down throttling —
+    /// can't bake its lateness into the next phase's timer). Scoped to one page; a human
+    /// transition to another page self-anchors.
+    @State private var pageAnchor: PageAnchor?
+
+    private struct PageAnchor {
+        let page: Int
+        let start: Date
+        let end: Date?
+    }
 
     private enum PageChangeCause { case auto, remote }
 
@@ -253,13 +259,13 @@ struct ExerciseProgressView: View {
             case .auto:
                 // Rest / timed-work auto-advance: record forward progress, but don't
                 // broadcast — the mirror reaches this point on its own synchronized timer.
-                clearRemoteAnchor()
+                // The chained anchor for the new page was set by `advance`, not cleared.
                 recordProgress(for: newPage)
             case .none:
                 // A human swipe. Swiping back from the first set to Ready wipes the run
                 // (only the adjacent 1→0 swipe resets — a stray far jump never does); any
                 // other page records forward progress. Human transitions are broadcast.
-                clearRemoteAnchor()
+                clearAnchor()
                 if showsReady && newPage == 0 && oldPage == base {
                     resetExercise()
                 } else {
@@ -333,9 +339,7 @@ struct ExerciseProgressView: View {
         knownStateIndex = frame.phase == .ready ? 0 : max(knownStateIndex, completedSets(for: target))
         // Anchor the target page's timer to the sender's wall clock, not local now, so the
         // displayed countdown matches regardless of how long the frame took to arrive.
-        remoteAnchorPage = target
-        remoteAnchorStart = frame.phaseStart
-        remoteAnchorEnd = frame.phaseEnd
+        pageAnchor = PageAnchor(page: target, start: frame.phaseStart, end: frame.phaseEnd)
         guard target != currentPage else { return }
         pageChangeCause = .remote
         withAnimation { currentPage = target }
@@ -357,17 +361,15 @@ struct ExerciseProgressView: View {
         if log.sets > setCount { setCount = log.sets }
         let target = base + min(max(0, completed), setCount - 1) * 2
         guard target > currentPage else { return }
-        clearRemoteAnchor()
+        clearAnchor()
         pageChangeCause = .remote
         withAnimation { currentPage = target }
     }
 
-    /// Drop the remote anchor when a local transition moves us off the remote-driven page,
-    /// so the next phase counts off local `now` (and a swipe back doesn't reuse a stale anchor).
-    private func clearRemoteAnchor() {
-        remoteAnchorPage = nil
-        remoteAnchorStart = nil
-        remoteAnchorEnd = nil
+    /// Drop the page anchor when a human transition moves us off the anchored page, so the
+    /// next phase counts off local `now` (and a swipe back doesn't reuse a stale anchor).
+    private func clearAnchor() {
+        pageAnchor = nil
     }
 
     /// Inverse of `liveSnapshot`'s page→frame mapping: a frame's phase/set → page index.
@@ -384,9 +386,8 @@ struct ExerciseProgressView: View {
     /// Build the live-run frame for a given page: phase, the set it pertains to, and the
     /// wall-clock anchors the mirror counts off. Count-down phases (rest, timed work, finish)
     /// carry an end anchor; a rep-based work set counts up and leaves it `nil`.
-    private func liveSnapshot(for page: Int) -> LiveProgress? {
+    private func liveSnapshot(for page: Int, at now: Date = Date()) -> LiveProgress? {
         guard let log else { return nil }
-        let now = Date()
 
         func frame(_ phase: LiveRunPhase, setIndex: Int, end: Date?) -> LiveProgress {
             LiveProgress(
@@ -433,10 +434,10 @@ struct ExerciseProgressView: View {
     @ViewBuilder
     private func page(for index: Int) -> some View {
         let isActive = index == currentPage
-        // Only the remote-driven page carries the sender's anchors; every other page (reached
-        // locally by swipe or auto-advance) counts off its own `now`.
-        let anchorStart = index == remoteAnchorPage ? remoteAnchorStart : nil
-        let anchorEnd = index == remoteAnchorPage ? remoteAnchorEnd : nil
+        // Only the anchored page (remote frame or chained auto-advance) carries anchors;
+        // every other page (reached by a human transition) counts off its own `now`.
+        let anchorStart = pageAnchor?.page == index ? pageAnchor?.start : nil
+        let anchorEnd = pageAnchor?.page == index ? pageAnchor?.end : nil
         if showsReady && index == 0 {
             ReadyPhaseView(summary: readySummary, onStart: start)
         } else {
@@ -461,8 +462,8 @@ struct ExerciseProgressView: View {
                         isActive: isActive,
                         anchorStart: anchorStart,
                         anchorEnd: anchorEnd
-                    ) {
-                        withAnimation { advance(from: index) }
+                    ) { end in
+                        withAnimation { advance(from: index, phaseEndedAt: end) }
                     }
                 } else {
                     // Rep-based work set — count up; the user swipes left when done.
@@ -483,8 +484,8 @@ struct ExerciseProgressView: View {
                     isActive: isActive,
                     anchorStart: anchorStart,
                     anchorEnd: anchorEnd
-                ) {
-                    withAnimation { advance(from: index) }
+                ) { end in
+                    withAnimation { advance(from: index, phaseEndedAt: end) }
                 }
             }
         }
@@ -498,11 +499,20 @@ struct ExerciseProgressView: View {
         withAnimation { currentPage = base }
     }
 
-    /// Programmatically move one page right (used by the rest auto-advance), guarding
-    /// against overrun if the user swiped away in the meantime. Tagged `.auto` so the page
-    /// observer records progress but doesn't broadcast it (the mirror auto-advances too).
-    private func advance(from index: Int) {
+    /// Programmatically move one page right when a countdown phase ends, guarding against
+    /// overrun if the user swiped away in the meantime. Tagged `.auto` so the page observer
+    /// records progress but doesn't broadcast it (the mirror auto-advances too).
+    ///
+    /// `phaseEndedAt` is the finished phase's *computed* end — the next phase began then,
+    /// not when this tick finally got runtime. Anchoring the next page there means a device
+    /// that slept through the boundary (throttled wrist-down ticker) shows the same timer
+    /// as one that crossed it on time — and a stack of missed boundaries fast-forwards
+    /// itself, since each chained countdown lands already-elapsed and advances on its own
+    /// next tick.
+    private func advance(from index: Int, phaseEndedAt end: Date) {
         guard currentPage == index, index + 1 < totalPages else { return }
+        pageAnchor = PageAnchor(page: index + 1, start: end,
+                                end: liveSnapshot(for: index + 1, at: end)?.phaseEnd)
         pageChangeCause = .auto
         currentPage = index + 1
     }
@@ -892,8 +902,10 @@ private struct CountdownPhaseView: View {
     /// so the remaining time — and the auto-advance at zero — line up across both devices.
     var anchorStart: Date? = nil
     var anchorEnd: Date? = nil
-    /// Invoked once the countdown reaches zero (auto-advance to the next page).
-    let onFinished: () -> Void
+    /// Invoked once the countdown reaches zero (auto-advance to the next page), passing the
+    /// phase's *computed* end so the next phase can anchor at the boundary itself — not at
+    /// whenever this tick got runtime (a device asleep at the boundary ticks late).
+    let onFinished: (Date) -> Void
 
     @State private var startDate = Date()
     @State private var endDate = Date()
@@ -919,7 +931,9 @@ private struct CountdownPhaseView: View {
         endDate = anchorEnd ?? startDate.addingTimeInterval(Double(max(1, seconds)))
         lastPingSecond = Int.max
         didFinish = false
-        if haptic { WorkoutHaptic.start.play() }
+        // No buzz for a chained catch-up page whose whole window already elapsed while
+        // the device slept — it advances again on its next tick.
+        if haptic, endDate > Date() { WorkoutHaptic.start.play() }
     }
 
     private func tick() {
@@ -931,8 +945,10 @@ private struct CountdownPhaseView: View {
             // Time's up — final cue and slide on. If the wrist was down the timer may have
             // stalled; this then fires on the first tick once the app gets runtime again.
             didFinish = true
-            WorkoutHaptic.stop.play()
-            onFinished()
+            // Buzz only for a boundary that just happened — fast-forwarding through
+            // boundaries that passed while the device slept stays silent.
+            if Date().timeIntervalSince(endDate) < 3 { WorkoutHaptic.stop.play() }
+            onFinished(endDate)
         } else if remaining <= 3 && remaining < lastPingSecond {
             // Once-per-second countdown ping for the final three seconds.
             lastPingSecond = remaining