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/Entities.swift

@@ -11,6 +11,16 @@ import SwiftData
 // SwiftData PersistentIdentifier). Computed helpers preserve the API the views
 // used against the old Core Data classes.
 
+extension PersistentModel {
+    /// True while this model is registered with a live context. Reading a persisted
+    /// property on a dead model traps, and any entity can die under a view that
+    /// retains it (remote delete, reconcile prune — the cache is rebuildable). Check
+    /// this before every such read: `isDeleted` alone misses the unregistered state
+    /// after the deletion saves, when `isDeleted` reads false again but reads still
+    /// trap (see `WorkoutDocument.init?(fromLive:)`).
+    var isLive: Bool { !isDeleted && modelContext != nil }
+}
+
 // MARK: - Routine
 
 @Model
@@ -26,6 +36,7 @@ final class Routine {
     var activityTypeRaw: Int = 0
     var restSeconds: Int?
     var autoAdvance: Bool?
+    var targetHeartRate: Int?
 
     @Relationship(deleteRule: .cascade, inverse: \Exercise.routine)
     var exercises: [Exercise] = []
@@ -152,6 +163,7 @@ final class Workout {
     var deletedLogIDs: [String: Date]?
     var restSeconds: Int?
     var autoAdvance: Bool?
+    var targetHeartRate: Int?
 
     @Relationship(deleteRule: .cascade, inverse: \WorkoutLog.workout)
     var logs: [WorkoutLog] = []