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/Connectivity/LiveRunState.swift

@@ -34,6 +34,26 @@ final class LiveRunState {
         return c
     }
 
+    /// Latest live heart-rate sample forwarded by the watch (bpm), or `nil` when none is
+    /// fresh. Device-level, not run-scoped — it's the wearer's pulse. Auto-clears after
+    /// `hrStaleAfter` without a new sample, so the UI never shows a frozen number after
+    /// the watch stops sending (session ended, unreachable).
+    private(set) var heartRate: Double?
+
+    private var heartRateExpiry: Task<Void, Never>?
+    private static let hrStaleAfter: Duration = .seconds(30)
+
+    /// Apply an incoming heart-rate sample and (re)arm its staleness expiry.
+    func applyHeartRate(_ bpm: Double) {
+        heartRate = bpm
+        heartRateExpiry?.cancel()
+        heartRateExpiry = Task { [weak self] in
+            try? await Task.sleep(for: Self.hrStaleAfter)
+            guard !Task.isCancelled else { return }
+            self?.heartRate = nil
+        }
+    }
+
     /// Apply an incoming frame, dropping a stale (or redelivered) one for the same run.
     func apply(_ frame: LiveProgress) {
         if let c = current, c.logID == frame.logID, !frame.isNewer(than: c) { return }