End the watch workout session from durable state, not a view

The HKWorkoutSession that keeps the watch app foregrounded was ended only
by ActiveWorkoutGateView's onChange(of: activeWorkouts) — a view-level side
effect. When a run ended from the phone while the watch app was
backgrounded (kept alive only by the session) or torn down and rebuilt with
an already-empty list, that onChange never fired: the session leaked (the
app kept re-foregrounding on every wrist raise) and finishAndSave() never
ran, so the HR/energy summary was neither saved to Health nor forwarded.

Move session-end off the view into a long-lived WorkoutSessionCoordinator
owned by WatchAppServices, driven by a new bridge.onWorkoutsChanged
callback fired after every authoritative cache mutation (phone push or the
watch's own optimistic edit). The decision is a pure SessionEndPlanner
seam (mirrors WatchCacheApplier): a running session ends only on a genuine
non-empty -> empty transition of the active set, so the launch race (session
running before the run doc syncs) resolves to .none and never discards a
run we haven't heard about yet. Same move as the live-mirror's
repairFromDurable, one layer down.

Watch-only; no schema or wire change. SessionEndPlannerTests pins the
decision table; the OS-initiated-end path (system ends the session itself)
stays a documented residual in PLAN-watch-session-end.md.

Workouts Watch App/Views/ActiveWorkoutGateView.swift

@@ -18,7 +18,6 @@ import SwiftData
 /// exercise list.
 struct ActiveWorkoutGateView: View {
     @Environment(WatchConnectivityBridge.self) private var bridge
-    @Environment(WorkoutSessionManager.self) private var sessionManager
 
     @Query(sort: \Workout.start, order: .reverse) private var workouts: [Workout]
     @Query private var splits: [Split]
@@ -72,13 +71,6 @@ struct ActiveWorkoutGateView: View {
             // Nothing to run yet — pull fresh state in case the phone just started one.
             if activeWorkouts.isEmpty { bridge.requestSync() }
         }
-        .onChange(of: activeWorkouts.map(\.id)) { previouslyActive, nowActive in
-            // The active list just emptied — the run either completed or was discarded.
-            // Route the HealthKit session accordingly.
-            if nowActive.isEmpty, !previouslyActive.isEmpty {
-                endSession(previouslyActive: previouslyActive)
-            }
-        }
         // The phone just entered (or left) an editor — if we're inside the now-locked run,
         // pop back to the gate so re-entry rebuilds a fresh working copy. Also pop if the run
         // we're inside was pruned (discarded/deleted on the phone, or aged out of the push).
@@ -98,35 +90,6 @@ struct ActiveWorkoutGateView: View {
         }
     }
 
-    /// The active list emptied: decide whether the run completed (save it to Health,
-    /// attach the captured metrics, and forward them to the phone) or was discarded
-    /// (throw the session data away). A completed run stays in the cache as `.completed`;
-    /// a discarded one is tombstoned on the phone and pruned from the cache, so its
-    /// absence from `workouts` is the discard signal.
-    private func endSession(previouslyActive ids: [String]) {
-        let finished = ids
-            .compactMap { id in workouts.first { $0.id == id } }
-            .first { $0.status == .completed }
-
-        guard let finished else {
-            // Nothing survived as completed → the run was discarded. Don't save it.
-            sessionManager.discard()
-            return
-        }
-
-        let doc = WorkoutDocument(from: finished)
-        Task {
-            guard var metrics = await sessionManager.finishAndSave() else { return }
-            metrics.totalVolume = WorkoutVolume.total(doc.logs)
-            var updated = doc
-            updated.metrics = metrics
-            updated.updatedAt = Date()
-            // Carry the captured metrics back to the phone (and thus iCloud) over the
-            // existing workout-update path.
-            bridge.update(workout: updated)
-        }
-    }
-
     /// If the run we're currently navigated into is no longer available — pruned from the
     /// cache (discarded/deleted on the phone, or aged out of the pushed set), or locked
     /// because the phone took over editing it — pop back to the gate.