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

Workouts Watch App/WatchAppDelegate.swift

@@ -25,6 +25,9 @@ final class WatchAppDelegate: NSObject, WKApplicationDelegate {
         if ScreenshotSeed.isActive { return }
         #endif
         sessionManager.requestAuthorization()
+        // Re-adopt a session that outlived a crash or reboot — watchOS relaunches us
+        // expecting this call; without it the session is orphaned and its workout lost.
+        sessionManager.recover()
     }
 
     func handle(_ workoutConfiguration: HKWorkoutConfiguration) {