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/Documents.swift

@@ -35,6 +35,11 @@ struct RoutineDocument: Codable, Sendable, Equatable, Identifiable {
     /// which is preferable to quarantining the user's whole routine.
     var restSeconds: Int? = nil
     var autoAdvance: Bool? = nil
+    /// Target heart rate (bpm) for endurance routines — drives the live too-low /
+    /// too-high indicator during a run. Nil → no target. Only meaningful when the
+    /// `activityType` supports it (see `WorkoutActivityType.supportsHeartRateTarget`).
+    /// Optional and NOT schema-bumped, same rationale as the fields above.
+    var targetHeartRate: Int? = nil
 
     // Bumped 1→2 when the weight-reminder fields (`weightLastUpdated`,
     // `weightReminderWeeks`) and `category` were removed and `machineSettings` was
@@ -112,12 +117,13 @@ struct WorkoutDocument: Codable, Sendable, Equatable, Identifiable {
     /// period. See `WorkoutMergePlanner`.
     var deletedLogIDs: [String: Date]? = nil
 
-    /// Snapshot of the routine's rest length / flow flag at plan time (a running
-    /// workout has no live link to its routine, same reason sets/reps/weight are
-    /// snapshotted per log). Optional and not schema-bumped, same rationale as
-    /// `RoutineDocument.restSeconds`/`autoAdvance`.
+    /// Snapshot of the routine's rest length / flow flag / target heart rate at plan
+    /// time (a running workout has no live link to its routine, same reason
+    /// sets/reps/weight are snapshotted per log). Optional and not schema-bumped,
+    /// same rationale as `RoutineDocument.restSeconds`/`autoAdvance`.
     var restSeconds: Int? = nil
     var autoAdvance: Bool? = nil
+    var targetHeartRate: Int? = nil
 
     // PINNED CODING KEYS: the Swift properties `routineID`/`routineName` were renamed
     // from `splitID`/`splitName`, but their on-disk (and iPhone↔Watch wire) JSON keys
@@ -139,6 +145,7 @@ struct WorkoutDocument: Codable, Sendable, Equatable, Identifiable {
         case deletedLogIDs
         case restSeconds
         case autoAdvance
+        case targetHeartRate
     }
 
     // Bumped 1→2 when `metrics` was added: the captured HR/calorie data is