Make the watch own its workout-session lifecycle

The HKWorkoutSession could only ever be born via the phone's one-shot
startWatchApp handoff (BULLETPROOFING.md H1-H3): a dropped handoff, a
watch crash/reboot, or a run engaged manually on the wrist left the
whole workout sessionless - no heart rate, no Health save, app
suspending wrist-down - and "End Current & Start New" swallowed the
handoff against the old session's idempotency guard.

Now the coordinator self-starts a session on any reconcile that finds
an active run with none running (SessionEndPlanner.shouldStart /
runToStart, activity type from the run's split), recover() re-adopts a
crash-orphaned session at launch, and a system-ended session salvages
its Health save instead of dropping it. A .finish decided for a
session younger than 30s demotes to .discard so the stale-context
races can't save junk workouts attributed to the wrong run; parallel
completions now pick the survivor deterministically.

Claude-Session: https://claude.ai/code/session_01PVNBVKp5bcq52X722uMjwT

CHANGELOG.md

@@ -1,5 +1,13 @@
 **July 2026**
 
+The watch now starts recording on its own whenever a workout is active, so a missed hand-off from the iPhone no longer leaves a session without heart rate or Health credit.
+
+A workout recording now survives the watch app crashing or the watch restarting, reconnecting to the in-progress session instead of losing it.
+
+A recording cut short by watchOS itself is now still saved to Apple Health instead of being silently dropped.
+
+Ending a workout and immediately starting a new one no longer risks a stray seconds-long workout appearing in Apple Health.
+
 A new Cardio exercise and matching starter split let you log aerobic sessions, tracked on your Apple Watch as a cardio workout.
 
 A new Auto-Advance option lets a split flow hands-free from one exercise to the next, resting automatically between them.