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

Shared/Model/Mappers.swift

@@ -28,7 +28,8 @@ extension RoutineDocument {
                   createdAt: routine.createdAt, updatedAt: routine.updatedAt,
                   exercises: routine.exercisesArray.map(ExerciseDocument.init(from:)),
                   activityType: routine.activityTypeRaw,
-                  restSeconds: routine.restSeconds, autoAdvance: routine.autoAdvance)
+                  restSeconds: routine.restSeconds, autoAdvance: routine.autoAdvance,
+                  targetHeartRate: routine.targetHeartRate)
     }
 }
 
@@ -52,7 +53,8 @@ extension WorkoutDocument {
                   logs: workout.logsArray.map(WorkoutLogDocument.init(from:)),
                   metrics: workout.metrics,
                   deletedLogIDs: workout.deletedLogIDs,
-                  restSeconds: workout.restSeconds, autoAdvance: workout.autoAdvance)
+                  restSeconds: workout.restSeconds, autoAdvance: workout.autoAdvance,
+                  targetHeartRate: workout.targetHeartRate)
     }
 
     /// Maps a *live* cache entity to a document, or `nil` when that entity has already
@@ -69,7 +71,7 @@ extension WorkoutDocument {
     /// read still traps. A `@Model` retained across time (not freshly fetched) can reach
     /// this map in that state, so check both.
     init?(fromLive workout: Workout) {
-        guard !workout.isDeleted, workout.modelContext != nil else { return nil }
+        guard workout.isLive else { return nil }
         self.init(from: workout)
     }
 
@@ -140,6 +142,7 @@ enum CacheMapper {
         routine.activityTypeRaw = doc.activityType ?? 0
         routine.restSeconds = doc.restSeconds
         routine.autoAdvance = doc.autoAdvance
+        routine.targetHeartRate = doc.targetHeartRate
 
         let existing = Dictionary(routine.exercises.map { ($0.id, $0) }, uniquingKeysWith: { a, _ in a })
         var keep = Set<String>()
@@ -200,6 +203,7 @@ enum CacheMapper {
         workout.deletedLogIDs = doc.deletedLogIDs
         workout.restSeconds = doc.restSeconds
         workout.autoAdvance = doc.autoAdvance
+        workout.targetHeartRate = doc.targetHeartRate
 
         let existing = Dictionary(workout.logs.map { ($0.id, $0) }, uniquingKeysWith: { a, _ in a })
         var keep = Set<String>()