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

README.md

@@ -83,6 +83,11 @@ your own iCloud Drive.
   bidirectional: drive from either device — swipe ahead, finish a set, add one — and the
   other follows. Only *human* transitions are sent; automatic ones (a rest timer ending)
   advance both devices independently off shared start times, so they never fight.
+- **Target heart rate** — give a cardio routine (HIIT, cardio, cycling) a target bpm
+  and the run screen shows your live heart rate from the watch — on the watch itself
+  and streamed to a propped-up iPhone — tinted green in range, with an arrow cue to
+  push harder or ease off. Perfect for dialing in a treadmill incline and holding a
+  steady effort.
 - **Watch face complication** — a launcher complication you can place on any Apple
   Watch face; tap it to open the app. Available in the circular, corner, inline, and
   rectangular accessory slots.