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 Watch App/Views/ExerciseProgressView.swift

@@ -25,6 +25,12 @@ import WatchKit
 struct ExerciseProgressView: View {
     @Environment(\.dismiss) private var dismiss
 
+    /// Live HR straight off the session's builder — drives the target-HR readout when
+    /// this run carries a `targetHeartRate`. Optional so hosts without a session
+    /// manager in the environment (the screenshot root) render without it.
+    @Environment(WorkoutSessionManager.self) private var sessionManager: WorkoutSessionManager?
+    @Environment(\.isLuminanceReduced) private var dimmed
+
     /// The shared working workout document owned by the parent. We mutate the matching
     /// log in place and ask the parent to forward each change through the bridge —
     /// driving the UI from this doc (not the cache) avoids losing rapid edits to the
@@ -285,6 +291,9 @@ struct ExerciseProgressView: View {
                     Image(systemName: "xmark")
                 }
             }
+            ToolbarItem(placement: .topBarTrailing) {
+                heartRateReadout
+            }
         }
         .confirmationDialog("Cancel Exercise?", isPresented: $showingCancelConfirm, titleVisibility: .visible) {
             Button("Cancel Exercise", role: .destructive) { dismiss() }
@@ -364,6 +373,26 @@ struct ExerciseProgressView: View {
         }
     }
 
+    // MARK: - Live heart rate vs. target
+
+    /// Compact live HR against this run's target (cardio routines only): bpm tinted by
+    /// band with a cue arrow — ease off (down) or push harder (up). Renders nothing
+    /// unless the run carries a target and a sample exists; dims under Always-On like
+    /// the timers.
+    @ViewBuilder
+    private var heartRateReadout: some View {
+        if let target = doc.targetHeartRate, let bpm = sessionManager?.currentHeartRate {
+            let band = HeartRateBand(bpm: bpm, target: target)
+            HStack(spacing: 2) {
+                Text("\(Int(bpm.rounded()))")
+                    .monospacedDigit()
+                Image(systemName: band.cueSymbol)
+            }
+            .font(.caption2.weight(.semibold))
+            .foregroundStyle(dimmed ? Color.secondary : band.tint)
+        }
+    }
+
     // MARK: - Live mirror
 
     /// Finish the initial-page restore, then either follow an in-progress remote driver or, if
@@ -749,6 +778,27 @@ private extension Color {
     static let restTimer = Color(white: 0.74)
 }
 
+// MARK: - Heart-rate band presentation
+
+private extension HeartRateBand {
+    /// The correction cue: below target → push harder (up), above → ease off (down).
+    var cueSymbol: String {
+        switch self {
+        case .low: "arrow.up"
+        case .inRange: "checkmark"
+        case .high: "arrow.down"
+        }
+    }
+
+    var tint: Color {
+        switch self {
+        case .low: .blue
+        case .inRange: .green
+        case .high: .orange
+        }
+    }
+}
+
 // MARK: - Phase Timer Layout
 
 /// The live timer a phase shows, described by its wall-clock anchors so `PhaseTimerLayout` can