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/Cardio/info.md

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+# Cardio
+
+A steady aerobic session — run in place, ride, row, or use the machine of your
+choice and just keep moving. It's logged as a single timed block; your Apple
+Watch records the real effort — heart rate, calories, and duration — as a
+cardio workout.
+
+- **Category:** Cardio
+- **Type:** Aerobic / conditioning
+- **Targets:** Heart, lungs, endurance
+- **Prescription:** 15–30 minutes at a conversational-to-brisk pace
+- **Defaults:** 1 × 1200 s
+
+## Setup
+
+Pick your mode — treadmill, bike, rower, or simply running in place — and let
+your watch start the session so the effort is tracked on your wrist.
+
+## Execution
+
+1. Ease in for the first minute or two, letting your heart rate climb.
+2. Settle into a steady pace you can hold for the whole block.
+3. Keep your breathing rhythmic and your shoulders relaxed.
+4. Cool down with an easy minute before you stop.
+
+## Cues
+
+- Aim for a pace where you could speak in short sentences, not full paragraphs.
+- Stay tall and loose — don't grip the rails or hunch over the machine.
+- Let the watch handle the numbers; just hold your effort steady.
+
+## Common Mistakes
+
+- Starting too hard and fading long before the time is up.
+- Leaning on the handrails, which quietly drops the real workload.
+- Skipping the warm-up and cool-down minutes at either end.
+
+## Progression
+
+Add minutes, lift the pace, or fold in short faster intervals to raise the
+challenge over time.