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/PhoneConnectivityBridge.swift

@@ -267,6 +267,10 @@ final class PhoneConnectivityBridge: NSObject {
             if let logID = dict[WCPayload.lpLogIDKey] as? String {
                 Task { @MainActor in self.liveRunState.end(logID: logID) }
             }
+        case WCPayload.liveHeartRateType:
+            if let sample = WCPayload.decodeLiveHeartRate(dict) {
+                Task { @MainActor in self.liveRunState.applyHeartRate(sample.bpm) }
+            }
         default:
             break
         }