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.

CHANGELOG.md

@@ -8,6 +8,8 @@ Each split can now set its own rest time, overriding the app-wide default in Set
 
 The Morning Wake-Up starter split is now a timed routine that auto-advances through every move.
 
+Splits can now be tagged Warm-Up or Stretching, and the built-in warm-up and stretch routines use them so they log correctly in Apple Health.
+
 Tap the speaker on any library exercise to hear its full instructions read aloud.
 
 Turn on Speak Exercise Cues in Settings to hear each exercise's setup and form cues spoken automatically when you start it, hands-free.