Add per-routine target heart rate with live in-run indicator

Endurance routines (HIIT, cardio, cycling) can carry an optional target bpm
(RoutineDocument.targetHeartRate, not schema-bumped — same preference-field
rationale as restSeconds/autoAdvance), snapshotted onto the WorkoutDocument at
plan time like the other pacing fields.

During a run, the watch streams its live HR sample to the phone over a new
best-effort liveHeartRate message — deliberately outside the LiveProgress
machinery (no version bump, no staging/retry; a gauge, not a record), throttled
to changed-bpm-or-10s in the watch bridge. LiveRunState holds the sample with a
30s staleness auto-clear so a dead stream never shows a frozen number.

Both run screens show the reading only when the run carries a target: the
phone's ExerciseProgressView as a top pill, the watch's in the top-trailing
toolbar slot, each tinted by a shared ±5 bpm HeartRateBand with an arrow cue to
push harder (low) or ease off (high) — e.g. dialing in a treadmill incline to
hold a steady effort.

Claude-Session: https://claude.ai/code/session_01Y7ZhkCYWNiTSAFhFCGnJ8n

Workouts Watch App/WorkoutSessionManager.swift

@@ -38,6 +38,11 @@ final class WorkoutSessionManager: NSObject {
     private(set) var currentHeartRate: Double?
     private(set) var currentActiveEnergyKcal: Double?
 
+    /// Fired with each fresh heart-rate sample while the session runs. The bridge hangs
+    /// off this to forward the reading to the phone (best-effort, display-only) — a
+    /// callback rather than observation so the sensor stream has exactly one push path.
+    var onHeartRateSample: ((Double) -> Void)?
+
     /// Seconds accumulated in each of 5 heart-rate zones (low→high). Built on the
     /// watch because it's the only place with the full HR stream. Empty/ignored when
     /// the user's age (hence max HR) is unknown.
@@ -182,7 +187,10 @@ final class WorkoutSessionManager: NSObject {
             let dt = now.timeIntervalSince(last)
             if dt > 0, dt < 60 { hrZoneSeconds[Self.zoneIndex(for: prevHR, maxHR: maxHeartRate)] += dt }
         }
-        if let newHR { currentHeartRate = newHR }
+        if let newHR {
+            currentHeartRate = newHR
+            onHeartRateSample?(newHR)
+        }
         lastHRSampleDate = now
 
         currentActiveEnergyKcal = builder?.statistics(for: HKQuantityType(.activeEnergyBurned))?