Add Warm-Up and Stretching activity types

Adds WorkoutActivityType.warmUp (HealthKit .preparationAndRecovery) and
.stretching (.flexibility), and retags the six starter splits that were all
mislabeled as Functional Strength:

- Warm-Up:    Upper Body Warm-Up, Lower Body Warm-Up, Morning Wake-Up
- Stretching: Morning Mobility, Full Body Stretch, Evening Stretch

The split editor's activity picker surfaces them automatically (CaseIterable).
Older app versions decode the new raw values as the default type — additive and
not schema-gated, so no quarantine.

Shared/HealthKit/HealthKitMapping.swift

@@ -24,6 +24,8 @@ extension WorkoutActivityType {
         case .coreTraining: .coreTraining
         case .cardio: .mixedCardio
         case .cycling: .cycling
+        case .warmUp: .preparationAndRecovery
+        case .stretching: .flexibility
         }
     }
 }