Add generic watch-tracked Cardio exercise and starter split

A single "Cardio" library entry (running-in-place figure, duration-logged)
plus a matching Cardio starter split tagged .cardio (HealthKit .mixedCardio),
so the Apple Watch records the real aerobic workout while the phone logs the
time — no per-machine cardio entries or schema changes.

Claude-Session: https://claude.ai/code/session_012qw2itfzKyEJ1HpsFt8Ex4

Scripts/generate_starter_splits.swift

@@ -177,6 +177,12 @@ let seeds: [Sp] = [
         Ex(name: "Cobra Stretch", sets: 1, reps: 8, load: 0),
         Ex(name: "Child's Pose", sets: 1, reps: 0, load: 2, dur: 45),
     ]),
+    // A single generic, watch-tracked cardio block: one duration exercise in a
+    // `.cardio` (→ HealthKit `.mixedCardio`) split. The phone logs the time; the
+    // watch records the real workout (heart rate, calories, duration).
+    Sp(name: "Cardio", color: "red", icon: "figure.run", activity: 4, ex: [
+        Ex(name: "Cardio", sets: 1, reps: 0, load: 2, dur: 1200),
+    ]),
 ]
 
 // MARK: - Emit