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

Workouts/Resources/StarterSplits/Cardio.split.json

@@ -0,0 +1,23 @@
+{
+  "activityType" : 4,
+  "color" : "red",
+  "createdAt" : "2020-01-01T00:00:00Z",
+  "exercises" : [
+    {
+      "durationSeconds" : 1200,
+      "id" : "01DXF6DT0032GKBRB638SQX8M3",
+      "loadType" : 2,
+      "name" : "Cardio",
+      "order" : 0,
+      "reps" : 0,
+      "sets" : 1,
+      "weight" : 0
+    }
+  ],
+  "id" : "01DXF6DT00BXJ6N63ZBX9M9VNH",
+  "name" : "Cardio",
+  "order" : 12,
+  "schemaVersion" : 3,
+  "systemImage" : "figure.run",
+  "updatedAt" : "2020-01-01T00:00:00Z"
+}