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

README.md

@@ -15,7 +15,7 @@ your own iCloud Drive.
   **Bodyweight Core** circuit with its own warm-up) appear automatically on first
   launch; editing one makes it your own copy, and deleted starters can be restored
   from Settings.
-- **Exercise library** — a bundled library of 47 exercises to populate your
+- **Exercise library** — a bundled library of 65 exercises to populate your
   splits, authored in `Exercise Library/` with per-exercise setup, cues,
   mistakes, progressions, and an anatomically-rigged 3D stick-figure visual
   system — covering every common movement pattern and gym station (see the
@@ -23,6 +23,10 @@ your own iCloud Drive.
   free-weight canon (deadlifts, squats, benches, rows, curls, raises), the
   cable staples, and the bodyweight staples (push-ups, pull-ups, dips,
   lunges, and the floor core set).
+- **Cardio** — a generic **Cardio** exercise and matching starter split for
+  aerobic work. Tag a split's activity type Cardio and your Apple Watch records
+  the real effort (heart rate, calories, duration) as a `.mixedCardio` workout,
+  while the phone logs a simple timed block — no per-machine tracking needed.
 - **Run a workout** — start a session from a split, then tap an exercise to run it
   as a paged flow: a **Ready?** lead-in, count-up work phases, count-down rests, and
   a **Finish** page — mirroring the Apple Watch. Swipe a row to mark it complete, or