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/Enums.swift

@@ -78,6 +78,31 @@ enum WorkoutActivityType: Int, CaseIterable, Codable, Sendable {
         case .mindAndBody: "figure.mind.and.body"
         }
     }
+
+    /// Whether a routine of this type can carry a target heart rate. Steady-state
+    /// effort calibration ("raise the incline until you hit 140") only makes sense
+    /// for the endurance types — a strength set's HR swings by design.
+    var supportsHeartRateTarget: Bool {
+        switch self {
+        case .hiit, .cardio, .cycling: true
+        default: false
+        }
+    }
+}
+
+/// Where a live heart-rate reading sits relative to a routine's target rate, with a
+/// ±`tolerance` bpm dead band so the indicator doesn't flap around the boundary.
+/// Shared by the phone and watch run screens so both classify identically.
+enum HeartRateBand: Sendable {
+    case low, inRange, high
+
+    static let tolerance = 5
+
+    init(bpm: Double, target: Int) {
+        if bpm < Double(target - Self.tolerance) { self = .low }
+        else if bpm > Double(target + Self.tolerance) { self = .high }
+        else { self = .inRange }
+    }
 }
 
 /// Where a workout's health metrics came from: real watch sensors, or — for records