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

Exercise Library/COVERAGE.md

@@ -74,8 +74,14 @@ entries) a user logging by that station's name will find.
   regression in their Progression sections, not separate picker entries.
 - **Dumbbell RDL, dumbbell lunge** — folded into Romanian Deadlift and Lunge
   ("barbell or dumbbells" in their pages), not separate entries.
-- **Kettlebells, T-bar row, pullover machine, cardio equipment** — out; not
-  strength stations found in most gyms (cardio is out of the app's scope).
+- **Kettlebells, T-bar row, pullover machine** — out; not strength stations
+  found in most gyms.
+- **Specific cardio machines (treadmill, bike, elliptical, rower, …)** — out as
+  individual picker entries. Cardio is covered instead by one generic **Cardio**
+  entry: a single duration-logged block whose split is tagged `.cardio` (→
+  HealthKit `.mixedCardio`), so the Apple Watch records the real workout (heart
+  rate, calories, duration) and the phone just logs the time. It reuses a
+  running-in-place figure rather than a per-machine rig.
 - **Morning wake-up mobility & stretching set** — landed. Dynamic warm-up
   drills (March in Place, Leg Swings, Neck Rolls, Arm Circles, Torso Twist,
   Standing Side Bend, Hip Circles) now fill the mobility row alongside Cat-Cow,